A semiconductor wafer is wetted with slurry by injecting the slurry into at least one channel which is provided in a wear ring, while the wear ring is holding the wafer and is pressed against a polishing pad. The calculation processing circuit 10 stores a plurality of tables 2a through 2c for different gaps between the target 3a and the magnetic sensors A and B, determines the gap based on the detection signals of the magnetic sensors A and B, and calculates the electrical angle of the detection signals based on the table corresponding to the determined gap. The activator compound is represented in one aspect by: . 28 or lower, that the duty ratio of the pulse frequency is controlled to be 0.