The present technology relates to a MOSFET protection circuit with which an increase in manufacturing cost associated with the securing of a sense region can be suppressed while suppressing a loss of main current, and relates to a protection circuit system provided with the protection circuit. The protection circuit is provided with: a first MOSFET(1) for power through which main current flows; an IGBT(5) which is connected in parallel with the first MOSFET and through which a shunt current from the main current flows; a sense resistor (6) connected in series with the IGBT; and a first control circuit (4) which, on the basis of the value of a voltage applied to the sense resistor, controls the gate voltage of the first MOSFET, wherein the ratio of the current value of the shunt current flowing through the IGBT to the current value of the main current flowing through the first MOSFET is not less than 0.018% and not more than 0.022%.
