Thank you for the ID. The species shouldn't have existed in Japan, but was found in Hokkaido recently...
It's part of a taxonomically problematic complex, so either H. mantegazzanum, H. persicum, or any of their allies, or even hybrids. All are horrible weeds and quite dangerous to human health. Sun exposure and sap of these species can give skin blisters lasting for many months, in some cases even years, and the wounds heal very slowly. That makes handling and removing the plants highly problematic. The seed set is enormous and the rhizomes are difficult to uproot.