There is provided a mold unit for cooling a preform that has been injection-molded, the preform having a bottomed shape and made of a resin, the mold unit including a core mold having an outer shape corresponding to an inner shape of the preform, and being insertable into the preform, in which a flow path for a temperature adjustment medium extending in an axial direction of the core mold is formed inside the core mold, the flow path is formed at a position displaced in a circumferential direction of the core mold, and a position of the flow path is adjustable in the circumferential direction.