A balancer includes a balancer housing having an annular channel defined therein, at least one mass movably disposed in the channel, at least one magnet coupled to one side of the balancer housing to restrain movement of the mass along the channel when rotational speed of a drum of the washing machine is within a predetermined range, and an inclined sidewall formed at an inner surface of the balancer housing to support the mass in a direction resisting centrifugal force applied to the mass during rotation of the drum.