PURPOSE: To reliably prevent cavities from being produced in a die-cast rotor by connecting a degassing groove to an air intake device through a pressure reducing room, connecting a pressure switch to the pressure reducing room and pressure injecting fused aluminum after the pressure of the pressure reducing room is lowered.

CONSTITUTION: A rotor plate 27 is inserted to a metal shaft 28 and a flange 28a is engaged with a recessed portion 25. Subsequently, three-piece tubular mold 29 covers around the outer surface of the rotor 27. Then, an anti-pouring gate side mold 30 is positioned on the upper end of the rotor plate 27 and a holder 31 is put on the anti-pouring gate side mold 30. And fused aluminum 21 is poured into a sleeve 22 and a gate plate 24 is put on the sleeve 22. And an electromagnetic valve of an air intake device 40 is opened. Consequently, gas of the pressure reducing room 41 is taken out and the pressure of the pressure reducing room 41 is lowered. In addition, gas in a mold is taken out and the pressure of the mold is lowered. When the pressure of the pressure reducing room 41 is lowered, a pressure switch 45 is operated and pushes up a plunger 23 by high pressure and the fused aluminum 21 is pressure injected through a pouring gate 24a of the gate plate.