To provide parallel operation controlling method and controller capable of parallelly operating two thermostatic devices even when power is fed to the two thermostatic devices through an earth leakage breaker with relatively small rated capacity.

These controlling method and controller are for parallelly operating the thermostatic devices 1 and 2. The thermostatic device 1 and 2 are provided with housing vessels 11 and 21 for housing objects, heaters 12 and 22 for heating the inside of the vessels and temperature sensors 13 and 33 for detecting temperature inside the vessels, respectively. Energization of the heater 22 is controlled so that temperature inside the housing vessel 21 may be prescribed lower limit temperature of higher and a prescribed upper limit temperature or lower on the basis of a detected temperature of the temperature sensor 23. Only when energization of the heater 22 is not performed, energization of the heater 12 is allowed to be performed, and energization of the heater 12 is controlled so that temperature inside the housing vessel 11 may be a prescribed lower limit temperature or higher and a prescribed higher limit temperature of lower on the basis of a detected temperature of the temperature sensor 13.