An output controller for a stirling engine is provided in a cooling system that causes common cooling water to flow through both the stirling engine and an internal combustion engine serving as a motive power source other than the stirling engine. The output controller for the stirling engine includes a temperature adjustment portion that adjusts a temperature of the cooling water supplied to the stirling engine. Specifically, the temperature adjustment portion includes a temperature adjustment valve capable of adjusting the temperature of the cooling water supplied to the stirling engine by switchably setting at least one of partial cooling paths and into a communication state.