In a semiconductor sensor having a membrane structure, the destruction of the membrane caused by the expansion or contraction of a fluid within a hollow part formed under the membrane while the sensor is in use is prevented. A semiconductor sensor 10 comprising a substrate 30 and a membrane 20 formed on the top surface thereof, in which the bottom of the substrate 30 and a mounting surface 50 on which the sensor 10 is mounted are bonded, has pressure difference adjusting means 22a to 22c for eliminating the difference in pressure of a fluid between an inside and an outside of a hollow part 34 while the sensor is in use.