Spontaneously in Japanese

Question

How can we say spontaneously in a natural way in Japanese?

Here the context: My German friend will spontaneously visit Okayama tomorrow. Because of that I want to cancel our running session tomorrow.

I want to express that someone decided something on short notice. It wasn't possible to predict it or plan in advance. In English I see this as a soft excuse for the cancelation mentioned in the example sentence. Spontaneously fulfills this purpose. Can a Japanese word do it too?

My (Japanese) language partner recommended the following: 自発的に --> Literally means spontanous. 彼は自分自身で岡山を訪れることを決めました。--> In the context of the example a good way of saying spontaneous is 自分自身. Can you confirm his recommendation?

Thank you in advance for your kind help :)

Answer 1

So you would like to (not to explicitly) express that you need to cancel your running session to meet your friend, but it was your friend's sudden decision of his/hers and you had neither any control nor prediction for that.

If so, 自発的に is not a good choice: 自発的に is said to mean spontaneously, but its focus is that the act was not forced and 自発的 doesn't have the meaning of impulsiveness. I'd say voluntarily is closer to 自発的に. 自分自身で is similar in this regard, it emphasizes that it was his own decision (but sounds more like, out of careful consideration).

Quoting a dictionary (精選版　日本国語大辞典）：　自発的 -- 自分からはたらきかけるさま．物事を自分から進んで行うさま．

My suggestions:

• 友達が（突然）岡山に来ることになりました．

-ることになった sounds like that it is decided somewhere else and you are only being affected by that.

• 友達が岡山に来ることにしたそうで，， (I was told a friend of mine decided to come to Okayama,,)

is more explicit.