I am unable to help for Japan but I can give you what i do in the south west of UK?

in mid February garden centres start to stock seed potatoes (guaranteed to be virus, infection and pest free) i buy mine in first week of april just as it starts to warm up at night.
I place them with the most eyes up in packets like egg cartons (also known as chiting) in a cool bright place (a window cill that isn't in sun all day) it will grow dark coloured shoots that will end up erupting into life when planted.)
while it's chitting I'd prepare the ground adding manure or more compost added to the ground.
it will take around a month to chit, so it will be end of april/early may then I would plant them and leave them for a several months.

for best results i have found is you need day temps of 15C and up and night temperatures of 7-10C and up for it to work well.