A magnetic disc cartridge includes a casing formed of upper and lower shell halves mated together, a disc-like recording medium supported for rotation in the casing and a liner fixed to each of the inner surfaces of the upper and lower shell halves by ultrasonic welding by use of an ultrasonic welding horn. The liner has an outer contour defined by an outer peripheral edge, a central opening defined by an inner peripheral edge, and a slit-like cutaway portion defined by a pair of linear edges extending between the outer peripheral edge and the inner peripheral edge. The ultrasonic welding horn is provided with a welding pattern in the form of a protrusion pattern formed on a welding face thereof over the entire welding area in which the ultrasonic welding horn is brought into contact with the liner, and the liner is welded to the inner surface of the shell half over the entire area of the liner along the welding pattern.