To suppress the reflection of an electromagnetic wave from a load in a plasma generation device 30 that generates electromagnetic wave plasma by emitting the electromagnetic wave to a combustion chamber 10 of an engine 20. The plasma generation device 30 includes an electromagnetic wave oscillator 33 that oscillates the electromagnetic wave, an antenna 15a for emitting the electromagnetic wave oscillated by the electromagnetic wave oscillator to the combustion chamber 10 of the engine 20, and a stub adjustment unit 52, 53. The stub 51 is provided on a transmission line 60 for electromagnetic wave from the electromagnetic wave oscillator 33 to the antenna 15a. While the engine 20 is operating, the stub adjustment unit 52, 53 adjusts a short circuit location on the stub 51 based on the intensity of a reflected wave of the electromagnetic wave reflected from the antenna 15a.