Cooling system for cooling interior of substantially airtight housing

A heater protective device of a cooling system includes a couple of thermostats and one thermal fuse. The thermostats are secured to opposing sides of one metal plate. When a temperature of each thermostat rises due to the heat conducted through the metal plate and substantially reaches 40 degrees Celsius or higher, an internal connection of the thermostat is turned off, i.e., is opened. When the temperature of the thermostat decreases substantially below 40 degrees Celsius, the internal contact of the thermostat is turned on, i.e., is closed. The metal plate is secured to a heat receiving bracket that is secured to a heater cover and that is heated upon receiving radiated heat from the electric heater. A thermal fuse extends through the metal plate in a thickness direction of the metal plate and is secured thereto.