A method of assembling an electric actuator comprising: a motor; a motor casing including the motor and having a closed end and an opening end having an outward flange having a inner corner; a shaft driven by the motor and extending from the opening end of the motor casing, said shaft having a worm wheel at en extended part; and a gear casing including the worm wheel of the shaft, said gear casing having an end face that faces the opening end of the motor casing, an annular projection being axially formed on the end face of the gear casing and having an outer circumferential surface, a vertical seat face extending from the annular projection outwards, said method comprises the steps of: putting an elastic O-ring into a space formed by the seat face of the gear casing, the outer circumferential surface of the annular projection and the inner corner of the outward flange of the motor casing; characterised by the steps of; elastically collapsing and deforming the O-ring by pressing force onto the inner corner of the motor casing; putting the O-ring into a gap (a) between the inner circumferential surface of the motor casing and the outer circumferential surface of the annular projection of the gear casing; generating a reaction force acting uniformly to press the inner corner of the motor casing outwards in a radial direction to make alignment to allow the gap (a) to become constant in a circumferential direction thereby achieving suitable alignment of the axes of the motor casing and the gear casing; and connecting the motor casing to the gear casing with a plurality of bolts in suitable alignment of axes of the motor casing and the gear casing owing to elastic collapse of the O-ring in the space. a motor; a motor casing including the motor and having a closed end and an opening end having an outward flange having a inner corner; a shaft driven by the motor and extending from the opening end of the motor casing, said shaft having a worm wheel at en extended part; and a gear casing including the worm wheel of the shaft, said gear casing having an end face that faces the opening end of the motor casing, an annular projection being axially formed on the end face of the gear casing and having an outer circumferential surface, a vertical seat face extending from the annular projection outwards, said method comprises the steps of: putting an elastic O-ring into a space formed by the seat face of the gear casing, the outer circumferential surface of the annular projection and the inner corner of the outward flange of the motor casing; characterised by the steps of; elastically collapsing and deforming the O-ring by pressing force onto the inner corner of the motor casing; putting the O-ring into a gap (a) between the inner circumferential surface of the motor casing and the outer circumferential surface of the annular projection of the gear casing; generating a reaction force acting uniformly to press the inner corner of the motor casing outwards in a radial direction to make alignment to allow the gap (a) to become constant in a circumferential direction thereby achieving suitable alignment of the axes of the motor casing and the gear casing; and connecting the motor casing to the gear casing with a plurality of bolts in suitable alignment of axes of the motor casing and the gear casing owing to elastic collapse of the O-ring in the space.