A washing machine comprising: a cabinet; a drum rotatably arranged within the cabinet; an annular recess provided in the drum; and a balancer to offset an unbalanced load caused within the drum during rotation of the drum, wherein the balancer comprises: a balancer housing mounted to the recess and having an annular channel therein; a plurality of masses movably disposed in the channel; at least one groove formed on an inner surface (111a, 111b, 111c) of the balancer housing and accommodating the masses so as to restrict the masses moving along the channel when the rotational speed of the drum is within a predetermined rotational speed range; and at least one magnet coupled with the rear surface of the balancer housing so as to restrict the masses together with the groove, wherein the groove comprises: first support portions provided at both ends thereof so as to support the masses in the circumferential and radial directions of the balancer housing; and a second support portion provided between the first support portions so as to support the masses in the radial direction of the balancer housing; wherein the second support portion comprises at least one flat surface, a first inclined surface and a second inclined surface, and wherein the first inclined surface and the second inclined surface are connected at both ends of the flat surface and are obliquely formed inward of the channel. a cabinet; a drum rotatably arranged within the cabinet; an annular recess provided in the drum; and a balancer to offset an unbalanced load caused within the drum during rotation of the drum, wherein the balancer comprises: a balancer housing mounted to the recess and having an annular channel therein; a plurality of masses movably disposed in the channel; at least one groove formed on an inner surface (111a, 111b, 111c) of the balancer housing and accommodating the masses so as to restrict the masses moving along the channel when the rotational speed of the drum is within a predetermined rotational speed range; and at least one magnet coupled with the rear surface of the balancer housing so as to restrict the masses together with the groove, wherein the groove comprises: first support portions provided at both ends thereof so as to support the masses in the circumferential and radial directions of the balancer housing; and a second support portion provided between the first support portions so as to support the masses in the radial direction of the balancer housing; wherein the second support portion comprises at least one flat surface, a first inclined surface and a second inclined surface, and wherein the first inclined surface and the second inclined surface are connected at both ends of the flat surface and are obliquely formed inward of the channel. a balancer housing mounted to the recess and having an annular channel therein; a plurality of masses movably disposed in the channel; at least one groove formed on an inner surface (111a, 111b, 111c) of the balancer housing and accommodating the masses so as to restrict the masses moving along the channel when the rotational speed of the drum is within a predetermined rotational speed range; and at least one magnet coupled with the rear surface of the balancer housing so as to restrict the masses together with the groove, wherein the groove comprises: first support portions provided at both ends thereof so as to support the masses in the circumferential and radial directions of the balancer housing; and a second support portion provided between the first support portions so as to support the masses in the radial direction of the balancer housing; wherein the second support portion comprises at least one flat surface, a first inclined surface and a second inclined surface, and wherein the first inclined surface and the second inclined surface are connected at both ends of the flat surface and are obliquely formed inward of the channel. first support portions provided at both ends thereof so as to support the masses in the circumferential and radial directions of the balancer housing; and a second support portion provided between the first support portions so as to support the masses in the radial direction of the balancer housing; wherein the second support portion comprises at least one flat surface, a first inclined surface and a second inclined surface, and wherein the first inclined surface and the second inclined surface are connected at both ends of the flat surface and are obliquely formed inward of the channel.