the valve section 22 to be seated on the valve seat 19 in this instance the gas vent valve 20 starts departing from the terraced portion 35c of the projection 35 for a time period until the gas vent valve 20 is fully closed the gases can be led to the outside through a gap disposed between the gas vent valve 20 and the terraced portion 35c of the projection 35 on the other hand a flow of the molten material forced into the first passage section 11 up to the facing side wall of the projection 35 is changed so as to follow along the side wall thereof and direct toward the end portion of the valve section 22 of the gas vent valve 20 thereby acting upon the concave portion 26 effectively and enhancing a sensitivity of the gas vent valve 20 this arrangement permits an ensured seating of the valve section 22 of the gas vent valve 20 on the valve seat 19 until the molten material reaches the valve seat 19 thereby improving accuracy of the timing at which the gas vent valve 20 is closed the fitting of the projection 35 with the concave portion 26 provides the space 36 which in turn is communicated only with the first branch passage sections 37a 37b and the second branch passage sections 38a 38b so that a peripheral portion of the end surface 22b of the valve section 22 is rendered smaller to the largest possible extent in area that is exposed to the first passage section 11 thus the molten material is prevented from leaking into the space 29 as much as possible through the mating surfaces between the peripheral portion of the valve section 22.