Disclosed are a charge sensitive amplifier capable of minimizing a variation in a signal voltage of an output signal by applying a bias direct current to a gate of a feedback transistor, and a radiation sensor including the same. According to the charge sensitive amplifier and the radiation sensor including the same, it is possible to minimize a variation in a signal voltage of a charge sensitive amplifier output signal by applying a current, which is formed by mirroring a current bias circuit designed to be insensitive to PVT variations, to a gate of a feedback transistor. Furthermore, it is possible to reduce a variation in charging time and enable high-speed sensing by charging the signal voltage to the level of a common voltage VCOM by using a constant current supplied through a bandgap reference (BGR) circuit.