An engine control device 13 that cannot output a control signal to an electromagnetic wave emission device 30 is used to emit electromagnetic wave at an appropriate timing from the device 30 to a combustion chamber 10. A signal processing device 40 is connected to the engine control device 13 that outputs an ignition signal for instructing an ignition device 12 of the engine 20 to ignite fuel air mixture in the combustion chamber 10 of the engine 20. The signal processing device 40, upon receiving the ignition signal, outputs to the electromagnetic wave emission device 30 an electromagnetic wave drive signal that determines based on the ignition signal an emission period, which is a period for the electromagnetic wave emission device 30 to emit an electromagnetic wave to the combustion chamber 10, so that an ignition operation is performed during the emission period of the electromagnetic wave.