Well, thanks to all the message on the forum, I was able to open my iDeco account, with Monex.
I saw a withdraw of 23,000 JPY last month and was quite worried to not see it on the JIS&T website, but as it was mentioned here and there, they just needed several weeks.
Now, I have a question though. I will share a picture. After all, I'm still discovering the tool, and there's only 23,000 JPY there, so it's not super sensitive.

So, if I understand this correctly, it tells me that I bough the fund for 20,066, the value is currently 20,344 so I made some benefit.
But the thing is that 23,000 were withdrawn from my bank account, so, where is the difference?