A period from a time point when a wafer W is carried into a housing 10 to a time point when the wafer W after being exposed is completely ready to be carried out is set as a single cycle. A time period before a next cycle is begun and after the single cycle is completed is referred to as a standby time period. When an illuminance in dummy light emission is set to be Id; an illuminance in exposure, Is; a time length of the dummy light emission, Td; and a time length of the exposure, Ts, by setting the Id to satisfy an expression of Id=(Tp/Td)·Iw−(Ts/Td)·Is, an average illuminance within the single cycle is maintained constant between substrates.