To measure brain activity of an active animal during vigilance from a variety of regions by providing a substrate, more than one contactor that is provided on the underside of the substrate and puncture the biological tissue of an animal, and a processing circuit that is formed on the top face of the substrate and processes a biological signal obtained by the contactors.

A biological signal sensor 1 is furnished with a substrate 2 placed on the surface of the cerebral tissue of an animal as subject, metal electrodes arranged on the underside thereof, a semiconductor chip 4 fixed to the top face and forms a processing circuit of biological signals obtained from the metal electrodes. Electrodes 41 are formed around the semiconductor chip 4 and connected to wires 21 in the surrounding of the top face of the substrate 2 through bonding wires 5, many of which are connected to metal electrodes 3 through the wires of the substrate 2. Therefore, as biological signals can be obtained from the deep parts of many regions of the biological tissue of the subject with many metal electrodes, and connected to the outside only with several signal wires, the sensor can follow reliably quick actions of the animal.