score:6
You're a French citizen going to work in Japan starting in October. In august, however, you want to visit without a visa for 2 weeks, and are concerned that immigration will invariably activate your working holiday visa.
I just called immigration at Narita Airport (+81-476-326-832 or +81-476-342-211) who said that this is not a problem.
UPDATE: evidently you experienced a language barrier trying to call them yourself (which doesn't surpise me tbh - luckily I speak Japanese myself). Call these numbers again (during Japanese daytime, when more people are around) and say "Konnichiwa, eigo no dekiru yakuin wa imasu ka kudasai?", which means "Hello, is there any English speaking officer here, please?" They should be able to connect you to someone
At immigration, if you see them flipping to the passport page with your visa, just tell them you're only visiting for tourism for two weeks this time, and that you will come back in October for the work. They will then place a visitor's entry stamp (actually a sticker) in the passport.
Upvote:2
I thought I might add to this as Iβm going through a similar thing. We live in Melbourne Australia and have just been approved for a working holiday visa, however we are going to Japan in two weeks for a short holiday. We intend to activate our WH visas in May 2018. We were worried about accidentally activating the WH visa so we contacted the embassy in Melbourne today who advised that this is not a problem - we simply need to explain at the airport in Japan that we do not want to activate our visas during this stay and wish to travel on a tourist visa. It didnβt seem to be a problem at all, but if youβre not from Melbourne you may need to check with your local consulate. Hope this helps