Were you just charged/arrested of the marijuana thing, or also convicted? You're not making it clear. If you were not convicted of the pot thing, and you're saying none of your convictions were drug-related or carried >1yr sentence, then you should be fine based on your info. I'll continue below, however, on assumption that you were convicted and that the conviction was expunged and sealed.

Whether/not Jpn can see expunged conviction from another country... I don't think anyone can answer that. A quick google search suggests that expunged record can still show up in some cases, that some US states have varying laws pertaining to this (such as the right to not disclose any convictions that have been expunged), etc. However, we are talking about Jpn here.

If you're going to be truthful with Jpn immigration, you're going to check "yes" to that landing card question, they'll ask you what the convictions were for, and you're going to divulge everything (including your marijuana conviction, not just the other stuff). I know you are looking for someone to chime in here and say, "I didn't disclose a drug offense and it was fine." But I really don't think you're going to get that type of feedback from anyone here. So ultimately it's either you take a chance and keep a lid on it (which I can't recommend), or you consult an attorney who's knowledgeable on this matter, or you disclose everything to Jpn consulate and see what they say (which of course puts you at risk for forever ending up on their black list).