Lunar calibration technique for Imager visible channel has been studied using Japanese Multi-functional Transport SATellite-2 (MTSAT-2) Imager data. For visible channel calibration, MTSAT-2 Imager has the special device that introduces sunlight into optics, and has been calibrated during post-launch testing. The lunar images taken by MTSAT-2 could also be absolutely calibrated and enable us to propagate the absolute calibration to other geostationary imagers by comparing lunar image brightness. As a result of this process, a lunar calibration equation was established. It was used to compare the lunar brightness and obtain the absolute calibration information. The equation applies to GMS, GOES and METEOSAT, and then some of them were compared with ISCCP calibration slope. They showed good agreement, but there were partial discrepancies. Additional calibrated lunar images are required in various lunar phase angles and various wavelengths to improve the geostationary lunar calibration technique. It is proposed that GOES-R (and MTSAT-2 follow-on) will acquire these additional lunar images with the calibration mechanism for visible channel, especially during post-launch testing. Preliminary results of the lunar calibration study are shown in this poster.