To improve fluidity of molding material in a cavity by applying ultrasonic vibration to the molding material in the cavity.

Approx. 80-95 wt.% or 50-90 vol.% of filler is dispersed and mixed in thermoplastic resin to obtain a molding material m. The molding material m as melted is injected into the cavity 2 in a mold 1 from a gate 5 through a spray 3 and a runner 4, and the cavity 2 is filled with the molding material m under a specified pressure. Ultrasonic waves generated by a ultrasonic oscillator 8 is applied as ultrasonic vibration to a movable die 1b through a horn 7, and this ultrasonic vibration is applied to the molding material m in the cavity 2. As a result, the fluidity of the molding material m is enhanced, a favorable magnet roller without molding failure is formed, and further the magnetic powder in the molding material m is oriented as desired without fail through application of magnetic fields.