An apparatus according to embodiments detects locations of faults in a multilayer semiconductor (MLS). The apparatus comprises a laser source that outputs a laser beam, an optical system that directs the laser beam selectively onto a target region in the MLS to generate an irradiated zone in the MLS, a stage and a scanner that control a relative position between the irradiated zone and the MLS so that the irradiated zone moves along the target region, a controller system that measures electrical signals or electrical signal changes induced by a temperature increase in the MLS, and identifies a location of the target region and locations of faults in the MLS based on the measured electrical signal or the measured electrical signal changes. The target region is made of a material of which thermal conductivity is higher than that of a material around the target region and has a structure penetrating from shallow layers to deep layers of the MLS.