To provide a signal processing circuit and a signal processing system of a photomultiplier tube for expanding a dynamic range by a simple circuit constitution.

In the signal processing circuit 20, by constituting a current mirror circuit by an operation amplifier Op, a transistor Q15, and resistors R11, R13 or the like, a current IL equivalent to an anode current Ia of the photomultiplier tube 11 is input into a log amplifier Lg, and by this log amplifier Lg, the current IL output in linear characteristics from the transistor Q15 is converted into a signal voltage Vout having logarithmic characteristics and output. By this, since the signal voltage Vout corresponding to the anode current Ia of the photomultiplier tube 11 is output in logarithmic characteristics from the log amplifier Lg, compared with the case in which such a voltage is output still in linear characteristics, all the more because of logarithmic conversion and compression, the dynamic range is expanded.