A surgical device incorporates a brush electrode for tissue ablation and other forms of electrosurgical treatment. The insulating layer is formed with an insulating opening exposing the terminal, and the terminal is formed in a pattern which is indented in a thickness direction. A calculation part calculates the offset of the magnetic sensor based on a plurality of the magnetic data sequentially captured by the capture part. At least three sides of the block may also include anti-shear receptacles.