A balancer of an engine includes a rod and a balancer piston. A first end portion of the rod includes a joining portion which is arranged to be joined to a crankshaft of the engine at a position that is eccentric relative to a rotation axis of the crankshaft. A second end portion of the rod is arranged to reciprocate inside a cylinder according to a rotation of the crankshaft. The balancer piston is fixed to the second end portion of the rod. The balancer piston is arranged to reciprocate inside the cylinder, while rocking between a contact state in which the balancer piston is in contact with an inner wall surface of the cylinder and a non-contact state in which the balancer piston is separated from the inner wall surface. The balancer piston includes a cylindrical outer peripheral portion that is curved so as to swell toward the inner wall surface. The outer peripheral portion is arranged such that lines of intersection with planes perpendicular or substantially perpendicular to a central axis of the balancer piston define circles. The outer peripheral portion includes a contact start portion which is arranged to come into contact first with the inner wall surface when the balancer piston switches from the non-contact state to the contact state. A distance from a center of a maximum outer diameter circle with the largest radius among the circles to the contact start portion is larger than the radius of the maximum outer diameter circle.