A method of painting a portion of a shell of a brake booster which is joined together by a caulking operation. A pair of members are disposed in opposing relationship to each other, and a shear caulking operation is applied to the outer periphery thereof at a plurality of locations, thereby joining the members together to define a shell. The shell is held so as to be rotatable about its axis, and a nozzle of a paint applicator is disposed in opposing relationship with the outer periphery of the shell. By intermittently rotating the shell through a given angle, paint is applied successively to only the caulked portions.