A jacket heater placed in adjacent to an object to be heated so as to heat and keep warm said object, which comprises: a heating element, an outer covering member that envelops and accommodates the heating element, wherein the outer covering member is configured to comprise a first shaped body that is provided on a side thereof facing the object, and a second shaped body that is provided on a side opposite from said side facing the object, the second shaped body is a resin shaped body that has a porous structure, and the second shaped body has larger thickness than the first shaped body has. Provided is a jacket heater capable of efficiently keeping warm an object heated by this configuration.