A thermally operated refrigerator includes a hot space absorbing a heat from an outer heat source at a high temperature, an ambient space having a first portion and a second portion, a heat accumulator disposed between the hot space and the first portion of the ambient space for establishing a first fluid circuit in which an amount of operating fluid is filled, a hot displacer whose reciprocal movement establishes a periodical movement of the operating fluid in the first fluid circuit for rejecting the heat absorbed at the hot space to the first portion of the ambient space, a cold space absorbing a heat from a load at a low temperature, a cooler disposed between the cold space and the second portion of the ambient space for establishing a second fluid circuit in which an amount of operating fluid is filled, a cold displacer whose reciprocal movement establishes a periodical movement of the operating fluid in the second fluid circuit for rejecting the heat absorbed at the cold space to the second portion of the ambient space, a mechanism for driving the hot displacer and the cold displacer concurrently, an additional ambient space having a variable volume and disposed between the first portion and the second portion of the ambient space.