To cause uniform infiltration in a thermoplastic resin surface in a short time using a small amount of a surface reforming material. A surface reforming method of a thermoplastic resin includes a step of opening a movable mold after a thermoplastic resin is molded in a cavity to form a gap between a portion of a molded product and a stationary mold, a step of introducing supercritical fluid, subcritical fluid, or high pressure gas including a surface reforming material into the gap, a step of narrowing the gap by closing the movable mold by a predetermined amount to prevent leaking of the introduced supercritical fluid, subcritical fluid, or high pressure gas from the gap, and of charging the supercritical fluid, the subcritical fluid, or the high pressure gas into a narrowed second gap, and a step of clamping and compressing the molded product.