To provide a heater which avoids inconvenience such as water break, etc., caused by air penetration into a circulation path.

This heater is equipped with sealed circulation path (24) which circulates a heat medium (hot water 6) to heating terminals (2 and 4), keeping it at specified pressure, a pressurizing means (pump 47), a circulation means (circulation pump 26), a control means (regulator 94), etc., and it keeps pressurized state by replenishing the heat medium after circulation of the heat medium, when the sealed circulation path gets in low pressure state by the attenuation of the heat medium, thus this prevents the water break.