Water leakage of a steam oven provided on an aircraft for heating ingredients is to be prevented.Two solenoid valves 6 and 7 are inserted in series in a water supply circuit of a steam oven. A power switch 12 supplies a 15 V direct current to the solenoid valves 6 and 7. A controller 1 transmits a command to driving circuits 4 and 5 of the solenoid valves according to a signal of a temperature sensor 10 in the oven. An output of a clock generator 14 is counted by a frequency counter and a time in which the valves are opened is counted. When count of a valve opening time becomes longer than a predetermined value, the power switch 12 of the solenoid valves is disconnected to prevent water leakage.
