A direct charge-to-digital converter (QDC) based on charge injection for an X-ray/gamma ray photon counting detector is introduced in this paper. The proposed circuit is a direct conversion circuit unlike conventional indirect circuits. Direct charge signal processing does not lose time because it accomplishes conversion and resets simultaneously and starts the conversion without waiting for all the photon energy to be converted into charge. This conversion behavior was verified by simulating a direct QDC circuit. Moreover, in the simulation, the effect of the dark current in the detector was taken into consideration and an appropriate compensation method for this was proposed. An experimental circuit was built and tested with discrete components to confirm coincidence with the simulated results. Direct conversion is simpler than indirect conversion, and the resultant circuit contains fewer electronic components than an indirect converter. This circuit is suitable for high-density imaging sensor design. In addition, this architecture ideally counts generated charges individually, which is valuable for the analysis of charge drift inside the detector.