我先问了日本大使馆，回复如下：
There is no requirement for the 6 months validity of your passport if you are travelling to Japan.
But, you are advised to please check with the Immigration of Singapore and the Airline you will be travelling with, on whether you are allowed to travel with your current passport, not meeting the 6 months validity.

所以passport validity的问题不是日本政府的concern，而是本地机场和海关。打电话问新航，新航回复我说这是ground chk in staff和当班custom officer的决定，不能保证，再加上agent给了否定的答复，所以决定按照华新网友的建议去ICA更新护照。
去了ICA，直接上二楼柜台，柜台人员听说我下周一就要飞，连机票都没有看，直接递给我application form，今天申请，拜五walk in拿新护照。价钱不变，70新币。