To provide an operating force assist device capable of realizing miniaturization, general-purpose property, and energy saving.

This operating force assist device has a device main body 11, a hydraulic pump 80 generating assist force, a motor 70 driving it, an oil tank 90, and a motor driving circuit 100. The device main body 11 has a main cylinder 12, a primary piston 30, a secondary piston 40, a seal valve body 50, and a piston switch contact part 60. The motor 70 stops usually and is started when the primary piston 30 presses the secondary piston 40.