A novel method to measure temporal development of the FEL power has been developed using a silicon bolometer, which response linearly to input over a wide range. The Si bolometer has the time resolution of ∼1 ms, which is much longer than the FEL macropulse of a few μs, so that it measures energy in the macropulse. The number of amplifications is changed by varying the pulse length of electrons from the gun of the linac and energy in the FEL macropulse is measured at a wavelength ∼100 μm with the detector and appropriate Teflon absorbers. The energy development of the FEL macropulse is measured over eight orders of magnitude from a very low power level close to the incoherent radiation up to the FEL power saturation. The temporal development of the FEL power is derived from the energy development as a function of the number of amplifications. Then the FEL gain is derived from the power development. We measured energy development in the macropulse and derived the FEL gain at some cavity lengths. The maximum FEL gain thus evaluated is about 56 percent, which agrees with calculation by the Super-Mode theory.