Method for controlling cooking appliance

A controlling method performed by a cooking appliance, the cooking appliance comprising a body having a cavity provided therein with a cooking space, a steam supply device to supply steam into the cooking space, a heating unit to heat an inside of the cooking space, a convection fan to generate air flow in the cooking space, and a controller, the method comprising operating the steam supply device and the heating unit to maintain a temperature in the cooking space at a boiling point of water or less,

• wherein the method further comprising:
• operating a first preheating step comprising operating the steam supply device with a first output and operating the heating unit with a second output for one period;
• and
• operating a cooking step comprising operating the steam supply device with a third output less than the first output and operating the heating unit with a fourth output less than the second output for another period, after completion of the first preheating step,
• wherein the heating unit comprises a convection heater disposed on a rear surface of the cavity in a way that the convection heater is adjacent to the convection fan, the method comprising operating only the convection heater among heaters in the cooking appliance in the first preheating step and the cooking step.