A magnetic tape cartridge made up of a cartridge case composed of an upper and lower cases welded together on the mating surfaces by ultrasonic welding with a plurality of energy directors, and a single reel of magnetic tape rotatalby retained in the case, in which the weld deposits formed by the energy directors may be prevented from sticking out of the mating surfaces. A plurality of energy directors provided on one of the mating surfaces have an elongated shape, and are disposed with the longitudinal direction substantially perpendicular to the direction along the side wall of the upper and lower cases.