An electrical power steering system is configured to assist steering by transmitting, through a belt, a rotational force of a driving pulley coaxially coupled to a rotational shaft of an electrical motor to a driven pulley coaxially coupled to a rotational member of a steering apparatus. The electrical power steering system includes a belt tension adjustment mechanism for adjusting a tension of the belt. The belt tension adjustment mechanism includes: a guide surface and an engagement surface, which are formed as mating surfaces of a housing of the steering apparatus and a casing of the electrical motor so as to engage with each other; and a connector capable of relatively moving the engagement surface along the guide surface due to a connection force for connecting the housing and the casing under a state in which the guide surface and the engagement surface engage with each other. The guide surface is inclined by a predetermined amount with respect to an axial line direction of the rotational shaft.