A dome cover (6) is manufactured by injection molding by steps of: forming a cavity (14) having the shape of the dome cover (6) between an upper mold (12) and a lower mold (13) for an injection molding die (11), providing a gate port (16) for injecting melt resin at a position (of the upper mold (12)) corresponding to the zenithal portion (15) of the dome cover (6), and injecting the melt resin into the cavity (14) through the gate port (16). The resin injected from the center portion (the portion corresponding to the zenithal portion (15) of the dome cover (6)) of the cavity (14) flows uniformly toward the circumference of the cavity (14). Therefore, variations in pressure and temperature of the resin in the cavity (14) are restrained and, consequently, molding irregularity of the manufactured dome cover (6) is restrained. According to the method of manufacturing the dome cover as described above, generation of the partial molding irregularity on the manufactured dome cover is restrained.