減少をする

You’ll usually see this paired with a subject and the particle が. The structure is simple: [Something] が 減少をする. If you want to describe *how* it's decreasing, you can add an adverb like 急激に (suddenly/rapidly) or 徐々に (gradually). For example, 人口が徐々に減少をする means 'the population is gradually decreasing.' It’s a very clean, logical structure. Because it’s a bit formal, it fits perfectly into presentations, school essays, or when you’re trying to sound like a sophisticated analyst on a podcast. You wouldn't really use this to tell your friend you have less candy than before—that would be like wearing a tuxedo to a backyard BBQ.

Imagine you’re a YouTuber checking your analytics. You might say, 視聴回数が減少をしています (The view count is decreasing). It sounds a lot more 'business-like' than just saying they went down. In Japan, you’ll hear this constantly on the news regarding the 人口減少 (population decline). It’s practically the national catchphrase of the 21st century there. You might also find it in a weather report: 降水量が減少をしました (The amount of rainfall decreased). Even in the world of gaming, if a certain weapon gets 'nerfed' and its usage rate drops, a strategy guide might note that its 使用率が減少をした (usage rate decreased).

Reach for this phrase when you want to sound objective and precise. It’s perfect for Zoom meetings when you need to explain why the marketing budget is being cut. It’s also the standard choice for academic writing or formal emails. If you’re writing a caption for a serious LinkedIn post about market trends, 減少をする will give you that 'I know what I'm talking about' vibe. It’s also great for reporting facts where you want to keep your personal feelings out of it. It’s the language of charts, tables, and serious adulting.

Avoid using this for physical size or casual, everyday shrinkage. If your favorite T-shirt got smaller in the dryer, don't say it 減少をした. Use 縮む (to shrink) instead. Also, avoid it in very casual settings with close friends. If you tell your best friend that the amount of pizza in the box 減少をした, they might think you’ve spent too much time reading economics textbooks. In casual chat, just use 減る (heru). It’s shorter, easier, and won’t make you sound like a robot trying to blend in with humans.

Learners often mix up physical size with numerical quantity.

Another common slip-up is using it for 'lowering' something yourself, like the volume on a TV.

Remember, 減少をする is usually about a trend or a count changing, not you manually turning a knob. It's also worth noting that while 減少する (without the を) is actually more common, adding the を adds a layer of emphasis on the *action* itself, but using it in the wrong context can make you sound a bit stiff.

The most common cousin is 減る (heru). This is the 'everyday' version. If your ice cream is disappearing, it’s 減る. Then there’s 低下する (teika suru), which means a decline in quality, level, or value—like your grades or your motivation on a Monday morning. 削減する (sakugen suru) is another one, but that specifically means 'to cut' or 'to reduce' something intentionally, like a company cutting costs to save money. Finally, 衰退する (suitai suru) is used for the decline of a whole civilization or an industry. Unless you're talking about the Roman Empire, stick to 減少.

You can tweak the verb する to fit the timing. 減少をしました is the past tense (it decreased), and 減少をしています is the continuous form (it is currently decreasing). If you want to express a 'will to decrease' (which sounds a bit weird for a trend, but works if you're a policy maker), you could say 減少をさせたい (I want to make it decrease). You can also add descriptors: 大幅な減少をする (to decrease significantly) or わずかに減少をする (to decrease slightly). These additions make your Japanese sound much more nuanced and professional.

Is 減少をする more formal than 減少する? Yes, adding を makes the noun stand out more as a deliberate action or specific phenomenon. Can I use it for my weight? You *could*, but people usually use 減量 (genryō) for weight loss. Is it okay for business emails? Absolutely, it’s highly recommended for reports. What’s the opposite? The opposite is 増加をする (zōka o suru), which means to increase. Does it work for emotions? Not really; use 薄れる (usureru) for feelings fading away. Can I use it for speed? Usually, 減速 (gensoku) is used for slowing down. Is it a common JLPT word? Yes, you'll see it around the N3 and N2 levels quite often.