A sandwich panel includes a reinforcement member having a recess formed to extend in a longitudinal direction over a predetermined length and open laterally. The thermoplastic resin core material has a rectangular flat board-like shape, and includes a protrusion which is disposed on a peripheral side surface portion thereof and which is capable of being laterally press-fit with respect to the recess. The protrusion has opposing surfaces respectively opposing mutually opposed inner surfaces of the recess, the opposing surfaces being oriented so as to protrude from the peripheral side surface portion. At least one of the opposing surfaces is provided with a plurality of protuberances disposed at an interval in the longitudinal direction. Each of the protuberances has a predetermined height from the opposing surfaces enabling the protuberances to tightly contact the inner surfaces of the recess at the time of press-fitting.