A measuring method that utilizes a bandpass filter and a measurement apparatus to measure an intensity of light having a predetermined wavelength among lights emitted from a light source, the bandpass filter transmitting the light having the predetermined wavelength, the measurement apparatus measuring an absolute intensity of an incident light includes the steps of measuring an output of the measurement apparatus continuously, stopping or starting an emission of the light source in the measuring step, calculating a first extreme value t→t0−0 and a second extreme value t→t0+0 in the output of the measurement apparatus at time t0 where t is time in the measuring step, and t0 is time when the emission of the light source stops; and calculating a difference between the first extreme value t→t0−0 and the second extreme value t→t0+0.