A mold clamping device of an injection molding machine supports a motor fixing member configured to fix a mold opening/closing motor to a rear platen, on a base frame through a support member and slide plates. When viewed from a direction of a drive shaft of the mold opening/closing motor, the width of the support member is formed to progressively increase outwardly from the motor fixing member toward the base frame, and on a portion of the support member that is located on the base frame side, the slide plates are provided at at least both ends of the portion in a width direction of the support member.