In the any photo-imaging systems, the tone reproduction on the output is the one of the most essential function.In the traditional silver halide photography, so called analog photography, the tonal characteristics of the system is evaluated by tone reproduction curve of the Camera-Through Sensitometory (CTS) technique. In the traditional system, the tonal quality of photo-image depends mainly on the medium and its developing process. Although the contribution of camera is not important factor to that quality.In the current electronic digital photography, the tone characteristics will be evaluated by similar method to the CTS, however the image processing function of the digital steel camera (CTS) induces the remarkable effect to that quality. Thus the tonal characteristics will be affected by DSC itself. The signal career of photo image in the digital system is the image data to be more definite, detectable and electronically controllable than that in the analog system.This paper discusses the electronic direct evaluating method for the tone reproduction of digital photo-imaging system.