A washing machine comprising: a rear panel configured to be mounted on a wall; a tub supported by the rear panel and configured to contain wash water; a front panel having a first opening therein, on or in front of the tub; a rotatable drum in the tub; a driving unit providing power to the drum; a cover on the rear panel and covering the tub and the front panel; a water supply device configured to supply wash water to the tub; and a drain device having a drain filter in a wash water discharge path, the drain filter being configured to filter foreign matter in the wash water, characterized in that the drain filter comprises a filter main body communicating with a drain hole in the tub, forming a flow path to discharge wash water outside of the rear panel or the cover, the filter main body having a bottom side exposed through the cover, a filter cap unit opening to expose the drain filter and closing to cover the drain filter, and a cap coupling portion to which the filter cap unit is assembled, wherein the filter cap unit comprises: a main body coupling portion rotatably coupled to the filter main body and blocking or covering an opening in the filter main body; and a filter portion protruding at one side of the main body coupling portion, in the filter main body across a wash water flow path inside the filter main body, and configured to filter foreign matter in the wash water, wherein the main body coupling portion includes first and second coupling portions (332, 333) and a concave groove portion, wherein the first coupling portion extends along the outer surface of the filter cap unit, between one of a pair of guide bumps and an assembling protrusion provided on the cap coupling portion, and may be inserted into one of the pair of guide bumps, and wherein the second coupling portion has a larger width than a distance between said one guide bump and the assembling protrusion, may face the first coupling portion, and be inserted into the other of the pair of guide bumps. a rear panel configured to be mounted on a wall; a tub supported by the rear panel and configured to contain wash water; a front panel having a first opening therein, on or in front of the tub; a rotatable drum in the tub; a driving unit providing power to the drum; a cover on the rear panel and covering the tub and the front panel; a water supply device configured to supply wash water to the tub; and a drain device having a drain filter in a wash water discharge path, the drain filter being configured to filter foreign matter in the wash water, characterized in that the drain filter comprises a filter main body communicating with a drain hole in the tub, forming a flow path to discharge wash water outside of the rear panel or the cover, the filter main body having a bottom side exposed through the cover, a filter cap unit opening to expose the drain filter and closing to cover the drain filter, and a cap coupling portion to which the filter cap unit is assembled, wherein the filter cap unit comprises: a main body coupling portion rotatably coupled to the filter main body and blocking or covering an opening in the filter main body; and a filter portion protruding at one side of the main body coupling portion, in the filter main body across a wash water flow path inside the filter main body, and configured to filter foreign matter in the wash water, wherein the main body coupling portion includes first and second coupling portions (332, 333) and a concave groove portion, wherein the first coupling portion extends along the outer surface of the filter cap unit, between one of a pair of guide bumps and an assembling protrusion provided on the cap coupling portion, and may be inserted into one of the pair of guide bumps, and wherein the second coupling portion has a larger width than a distance between said one guide bump and the assembling protrusion, may face the first coupling portion, and be inserted into the other of the pair of guide bumps. a main body coupling portion rotatably coupled to the filter main body and blocking or covering an opening in the filter main body; and a filter portion protruding at one side of the main body coupling portion, in the filter main body across a wash water flow path inside the filter main body, and configured to filter foreign matter in the wash water, wherein the main body coupling portion includes first and second coupling portions (332, 333) and a concave groove portion, wherein the first coupling portion extends along the outer surface of the filter cap unit, between one of a pair of guide bumps and an assembling protrusion provided on the cap coupling portion, and may be inserted into one of the pair of guide bumps, and wherein the second coupling portion has a larger width than a distance between said one guide bump and the assembling protrusion, may face the first coupling portion, and be inserted into the other of the pair of guide bumps.