Penman-type evapotranspiration (ET) methods are widely used in irrigation water management and water resource planning. To determine the daily reference ET using Penman-type methods, net longwave radiation must be estimated through an empirical net longwave radiation equation based on meteorological data. This paper presents the coefficients of the net longwave radiation equation calibrated using highly accurate data observed at the Tateno observatory, which belongs to the Baseline Surface Radiation Network (BSRN). The daily net longwave radiation calculated with these calibrated coefficients is in better agreement with the observed data as compared with those of the original Penman–Monteith equation in the Food and Agriculture Organization (FAO) Irrigation and Drainage Paper no. 56 (FAO56-PM). In particular, the FAO56-PM cloudiness factor coefficients are more effectively calibrated in this study. The annual mean reference ET calculated through the calibrated FAO56-PM corresponds to approximately 90% of the values calculated using the original coefficients.