Provided is a core material of a vehicular seat cushion, the core material comprising a thermoplastic resin expanded beads molded article that has little warping and exceptional dimensional precision and that has a reinforcing member integrated therewith. A core material of a vehicular seat cushion, the core material comprising: a thermoplastic resin expanded beads molded article 2 that has a substantially rectangular shape in a top view, the rectangular shape having a long side and a short side; and an annular reinforcing member 3 that is insert-molded in the peripheral edge part of the expanded beads molded article, wherein: a lightening part 4 is formed in the expanded beads molded article, the lightening part passing through in the thickness direction; and a slit 7 is formed in at least one corner of the lightening part, the slit being oriented outward from the expanded beads molded article from the vicinity of the corner part.