A lens driving device has a motor mounted to a plate, a lead screw connected to the motor for undergoing rotation about an axial line thereof, a nut threadingly engaged with the lead screw, and a casing mounted to the plate for housing the lead screw and the nut. The nut has a convex protruding portion protruding outwardly from an outer peripheral surface of the nut. The casing has a fitting groove linearly formed in the casing in a direction of the axial line for receiving the protruding portion in a state in which the protruding portion is fitted in the fitting groove so as to regulate rotation of the nut, and for movably guiding the protruding portion in the fitted state in the direction of the axial line to move the nut back and forth along the lead screw. At least one of the casing and the protruding portion of the nut has a tapered surface formed so as to forcibly guide the protruding portion toward the fitting groove to gradually fit the protruding portion into the fitting groove when the casing and the plate are mounted together.