A new species of heteromorph ammonoid Eubostrychoceras valdelaxum sp. nov. is described from the Platyceramus japonicus Zone (the lowermost Campanian) of the Haboro and Mikasa areas in Hokkaido, northern Japan. The most notable characteristic of E. valdelaxum sp. nov. is its extremely longitudinally elongated helical whorls. The new species is clearly distinguished from other species by this characteristic. The shell ornamentation and whorl expansion rate of the new species resemble most closely those of E. japonicum. Considering the stratigraphic relationships, it is the most reasonable conclusion that E. valdelaxum sp. nov. evolved from E. japonicum. The variation of body chamber size in E. valdelaxum sp. nov. might be dimorphism.