An opening/closing assist device includes a cam fixed to a first member, which is one of an opening/closing body and a coupling subject of the opening/closing body, a contact element located on a second member, which is the other one of the opening/closing body and the coupling subject, and a damper stay. The damper stay includes a first end rotatably connected to the first member and a second end rotatably connected to the contact element. The contact element is maintained in the state of being pressed against the cam by means of the elastic force of the damper stay. The cam has a shape that is set so that a moment generated at the cam based on the elastic force applied by the contact element is generated so that a moment required to rotate the opening/closing body is in accordance with operation characteristics required for the opening/closing body.