The effective energy of diagnostic X-rays is important for quality assurance and quality control purposes. However, the half-value layer (HVL), which is necessary to evaluate the effective energy, has not been ascertained because of the high cost of the ionization chamber (IC) required and because the measurements are time-consuming and complicated. To solve these problems, a method using radiochromic film (RCF) and step-shaped aluminum (SSAl) filters has been developed. Gafchromic EBT (GAF-EBT), which has a weak energy dependence, was used as the RCF. The X-ray tube voltage for HVL was measured as 120 kV. The SSAl filter area, the full exposure area, and the non-exposure area were set on the GAF-EBT so as to obtain correct data. The density ratio of the SSAl filter area was calculated using the densities of the full exposure area and the non-exposure area. The GAF-EBT was scanned using a flat-bed scanner before and after exposure. To remove the image acquisition error of the flat-bed scanner, the scanning image of the GAF-EBT before exposure was subtracted from it after exposure. The HVL was evaluated using the density attenuation ratio of the GAF-EBT. Then, the effective energies obtained using the GAF-EBT and the IC were compared. The HVL and the effective energy with X-ray tube voltage of 120 kV using the GAF-EBT were 4.56 mm and 41.0 keV, respectively. The difference ratio of the effective energy between the GAF-EBT and IC methods was 5.9%. The effective energy can be measured using GAF-EBT easily and with high precision. The measurement time using GAF-EBT is extremely short compared with the IC method. Moreover, the effective energy can be measured inexpensively using the GAFEBT.