A regenerator accumulates cooling generated by expansion of refrigerant gas, and the regenerator includes a regenerator material which is made of a nonmagnetic material, a regenerator material which is made of a magnetic material, a container which includes a high temperature end and a low temperature end, and which accommodates the regenerator material made of the nonmagnetic material at the high temperature end side and the regenerator material made of the magnetic material at the low temperature end side. The container further accommodates an insertion member which narrows a passage area of the refrigerant gas flowing to a region accommodating the refrigerator material made of the magnetic material so that the passage area of the low temperature end side is narrower compared to the passage area of the high temperature end side.