The invention discloses a flame monitoring system for a burner of a gas appliance, and a control method for the gas appliance. The Flame monitoring system for a burner of a gas appliance, the flame monitoring system comprising a thermocouple (2) adapted for being arranged next to the burner and an electromagnetic gas valve adapted for opening or closing a gas passage to said burner, the electromagnetic gas valve being electrically connected with the thermocouple (2) and said electromagnetic gas valve keeping the gas passage open when it receives a specific current from the thermocouple (2). The flame monitoring system (1) also comprises a voltage meter (4) for measuring the voltage of the thermocouple (2), and interruption means (5) configured for interrupting current flow between the thermocouple (2) and the electromagnetic gas valve during a predetermined time so that the voltage meter (4) can measure the voltage of the thermocouple (2) in vacuum, said predetermined time being such that the electromagnetic gas valve keeps the gas passage open during the interruption by means of the actual inertia of the electromagnetic gas valve.