PURPOSE: To provide an auxiliary driving force controller for a power assist vehicle which can avoid adverse effects on control accuracy of auxiliary driving force due to the manufacture and an assembly error or secular change of a stepping force detector.

CONSTITUTION: In an auxiliary driving force controller of a power assist vehicle which is provided with a human power drive mechanism which drives drive wheels due to pedaling force inputted in a crankshaft and an auxiliary drive mechanism which drives drive wheels due to auxiliary driving force which corresponds to energy of pedaling force, a stepping force detection means 44 which detects energy of pedaling force, a detection means 21 when stepping force is not inputted which detects the condition in which pedaling force is not inputted, and an auxiliary driving force control means 21 which controls the auxiliary driving force by using a stepping force detected value when stepping force is not inputted as control reference value.