A washing machine having a drum and a balancer mounted to the drum to offset unbalanced load generated in the drum during rotation of the drum, the balancer comprising: a balancer housing having an annular channel defined therein, the balancer housing comprising a first housing opened at one side thereof; and a second housing to cover the first housing to define the annular channel, wherein the first housing comprises a first inner surface corresponding to an inner circumference of the first housing, and a second inner surface corresponding to an outer circumference of the first housing and being opposite to the first inner surface, the balancer housing further comprising a third inner surface connected between the first inner surface and second inner surface; at least one mass movably disposed in the channel; at least one magnet coupled to a rear surface of the balancer housing to restrain movement of the mass along the channel when rotational speed of the drum is within a predetermined range; at least one groove formed at an inner surface of the balancer housing to receive the mass; and an inclined sidewall formed at the second inner surface corresponding to the first inner surface in which the groove is formed, the inclined sidewall being provided at an angle with respect to the second and the third inner surfaces to support the mass in a direction resisting centrifugal force applied to the mass during rotation of the drum, the inclined sidewall comprising a section having a first inclination angle disposed between two sections of the inclined sidewall, each having a second inclination angle, different to the first inclination angle, first and second inclination angles being formed between the second inner surface and the inclined sidewall. a balancer housing having an annular channel defined therein, the balancer housing comprising a first housing opened at one side thereof; and a second housing to cover the first housing to define the annular channel, wherein the first housing comprises a first inner surface corresponding to an inner circumference of the first housing, and a second inner surface corresponding to an outer circumference of the first housing and being opposite to the first inner surface, the balancer housing further comprising a third inner surface connected between the first inner surface and second inner surface; at least one mass movably disposed in the channel; at least one magnet coupled to a rear surface of the balancer housing to restrain movement of the mass along the channel when rotational speed of the drum is within a predetermined range; at least one groove formed at an inner surface of the balancer housing to receive the mass; and an inclined sidewall formed at the second inner surface corresponding to the first inner surface in which the groove is formed, the inclined sidewall being provided at an angle with respect to the second and the third inner surfaces to support the mass in a direction resisting centrifugal force applied to the mass during rotation of the drum, the inclined sidewall comprising a section having a first inclination angle disposed between two sections of the inclined sidewall, each having a second inclination angle, different to the first inclination angle, first and second inclination angles being formed between the second inner surface and the inclined sidewall.