An insulation resistance detection circuit is coupled to a positive end (V+) and a negative end (V-) of a DC power source (Vdc), and is used to detect a positive insulation resistance (Rx) between the positive end (V+) and a ground point (G) and detect a negative insulation resistance (Ry) between the negative end (V-) and the ground point (G). A detection unit (DET) sets a first estimated resistance and a second estimated resistance, and acquires a first voltage (V1) based on turning on the switch (SW) and acquires a second voltage (V2) based on turning off the switch (SW). The detection unit (DET) calculates a third voltage (V3) and a fourth voltage (V4) according to the first estimated resistance and the second estimated resistance so as to detect the positive insulation resistance (Rx) and the negative insulation resistance (Ry) when the third voltage (V3) is equal to the first voltage (V1) and the fourth voltage (V4) is equal to the second voltage (V2).