An isolation region is provided around a sense part. The isolation region is provided to have a depth that suppresses spread of a region with an uneven current distribution, which occurs at a peripheral edge of the sense part. Thus, in the sense part, an influence of the region with the uneven current distribution can be suppressed. Since the current distribution can be set more even throughout the sense part, the on-resistance in the sense part can be set closer to its designed value. Thus, a current ratio corresponding to a cell ratio can be obtained as designed. Consequently, current detection accuracy is improved.