To provide a protective tube for a liquid-expansion thermostat, which appropriately covers and protects the liquid-expansion thermostat (liquid expansion diaphragm type automatic temperature regulator) that utilizes volume changes, responding to the temperature changes of a liquid.

The protective tube which covers and protects the liquid-expansion thermostat composed of a capillary tube and a temperature-sensitive element, has a flexible first pipe that contains the capillary tube therein, and a non-flexible second pipe that contains the temperature-sensitive element therein. The first pipe is connected with the second pipe so as to be in communication with each other.