The atmospheric transmittance of the energy transfer laser beam of LSSPS (Laser-type Space Solar Power System) is evaluated with a numerical model. The damping of the laser beam is one of the efficiency factors of the LSSPS. The numerical model computes atmospheric conditions include clouds and simulates the transmittance of the beam from the orbit of a stationary satellite. The simulated transmittance changes widely in time due to weather changes. The field observation of the direct solar irradiance also performed with a skyradiometer and evaluated the atmospheric transmittance of the beam from the observed irradiance. The good agreement of the simulated transmittance with the observed one shows the availability of the numerical model.