A motor mounting structure for an electric seat sliding device includes: a motor; first and second elastic support members arranged to be mounted on the first and second protruding portions of the motor; a bracket arranged to support the both ends of the motor through the first and second elastic support members; and the second elastic support member being arranged to be moved between a position at which the second elastic support member is engaged with the second guide portion, and a position where the second elastic support member is disengaged from the second guide portion, and where the second elastic support portion is mounted on the second protruding portion of the motor positioned at the initial assembly position.