An ejector apparatus moves a lift core extending through a core of a component of a resin molding mold to form an undercut portion in a molded piece and is installed so as to be capable of moving obliquely with respect to the core surface in a longitudinal direction. An ejector plate is arranged between the core and a base plate below the core, so as to be capable of moving up and down. A slide path is formed in the ejector plate extending in a direction in which a lower end of the lift core makes relative horizontal movement during ascent and descent of the lift core. A slide base is movably arranged in the slide path and coupled to the lower end of the lift core. A guide rod imparts a moving force within the slide path to the lower end of the lift core during ascent and descent of the ejector plate.