A surface property inspection device, comprising: an AC bridge circuit; an AC power supply for supplying AC power to the AC bridge circuit; a judgment device for making a pass/fail judgment of the surface treatment condition of a sample under inspection based on the output signal from the AC bridge circuit; and a frequency adjuster for adjusting and setting the frequency of AC power supplied from the AC power supply; wherein the AC bridge circuit includes a variable resistor constituted so that the split ratio (γ) between a first resistor and a second resistor is variable, a benchmark detector having a first magnetic sensor for detecting the magnetic properties of a benchmark sample in a good surface treatment condition, and an inspection detector having a second magnetic sensor, separate from the first magnetic sensor, for detecting the magnetic properties of a sample under inspection for which a pass/fail inspection of the surface treatment condition is to be made; and the first resistor, the second resistor, the benchmark detector, and the inspection detector constitute a bridge circuit; and wherein the judgment device is configured to judge the pass/fail status of the surface treatment condition of the sample under inspection based on the output signal from the AC bridge circuit in a state in which AC power is supplied to the AC bridge circuit, the first magnetic sensor is in contact or proximity with the benchmark sample, and the second magnetic sensor is in contact or proximity with the sample under inspection; and characterized in that in a state in which the first magnetic sensor detects the magnetic properties of the benchmark sample, and the second magnetic sensor detects the magnetic properties of a non-surface treated sample or a reference sample which is a sample in a poor surface treatment condition, the frequency adjuster is configured to set the frequency so that the amplitude of the output signal from the AC an AC bridge circuit; an AC power supply for supplying AC power to the AC bridge circuit; a judgment device for making a pass/fail judgment of the surface treatment condition of a sample under inspection based on the output signal from the AC bridge circuit; and a frequency adjuster for adjusting and setting the frequency of AC power supplied from the AC power supply; wherein the AC bridge circuit includes a variable resistor constituted so that the split ratio (γ) between a first resistor and a second resistor is variable, a benchmark detector having a first magnetic sensor for detecting the magnetic properties of a benchmark sample in a good surface treatment condition, and an inspection detector having a second magnetic sensor, separate from the first magnetic sensor, for detecting the magnetic properties of a sample under inspection for which a pass/fail inspection of the surface treatment condition is to be made; and the first resistor, the second resistor, the benchmark detector, and the inspection detector constitute a bridge circuit; and wherein the judgment device is configured to judge the pass/fail status of the surface treatment condition of the sample under inspection based on the output signal from the AC bridge circuit in a state in which AC power is supplied to the AC bridge circuit, the first magnetic sensor is in contact or proximity with the benchmark sample, and the second magnetic sensor is in contact or proximity with the sample under inspection; and characterized in that in a state in which the first magnetic sensor detects the magnetic properties of the benchmark sample, and the second magnetic sensor detects the magnetic properties of a non-surface treated sample or a reference sample which is a sample in a poor surface treatment condition, the frequency adjuster is configured to set the frequency so that the amplitude of the output signal from the AC