A tub of a washing machine includes: a first case and a second case welded to each other; a first coupling surface formed along a circumference of the first case facing the second case; a second coupling surface formed along a circumference of the second case facing the first case and bonded to the first coupling surface; and a coupling protrusion that protrudes along the first coupling surface and includes a protruding end portion welded to the second coupling surface. The coupling protrusion includes: a main coupling protrusion protruding along the first coupling surface; and an auxiliary coupling protrusion protruding along the first coupling surface and outwardly spaced apart from the main coupling protrusion. Further, an opening for communicating a space between the auxiliary coupling protrusion and the main coupling protrusion to an outside of the tub is defined in a bottom of the tub.