PURPOSE: To manufacture the frame described in the title having high rigidity by supplying a fiber reinforced synthetic resin material including long fiber which is heated to a heat distortion temperature into the spaces formed by split upper and lower molds and sliding molds which move into the side parts thereby forming a gate shape.

CONSTITUTION: Recessed parts 11b formed in the inside of the movable upper mold 11 and recessed parts 12b are symetrically formed in the inside of the fixed lower mold 12. Each sliding mold 13, wherein thickness of a protruded tip part 13a is reduced and which can be slidden in the right and left directions, is provided between said combined recessed parts 11a and 12a of the upper and lower molds 11 and 12. The fiber reinforced synthetic resin material W including the long fiber which is heated to the heat distortion temperature is applied between the upper and lower molds 11 and 12. Then the upper mold is lowered and each sliding mold 13 is slidden inward to the recessed parts 11b and 12b. The material W is held and compressed by the inside of the recessed parts 11b and 12b and the protruded part 13a. Thus, the sheet frame, which has a channel shaped cross section and comprises parallel flange parts 5d and an end wall 5e, can be obtained.