An extension assisting device that can assist mobility of a joint is provided. An extension assisting device (10) has: a thigh arm (16) that engages with a front portion of a thigh (P3) of a user (P)； and an artificial muscle (20) that, by being operated, applies force toward a rear side from the thigh arm (16) to the thigh (P3) of the user (P), and displaces the thigh (P3) of the user (P) toward the rear side relative to an upper body (P2).
