A biosensor not requiring washing of an unbonded label molecular by analyzing an object to be measured such as an antigen, an antibody, a DNA, and an RNA through magnetic field sensing. The biosensor is small in size, low in price, and high in sensing accuracy. Semiconductor Hall devices are arrayed two-dimensionally on the bottoms of recesses in the surface of a sensor chip to which bonded is a magnetic molecule with which a magnetic particle is labeled so as to sense the magnetic field produced along the sensor surface of the sensor chip. The surface area of each semiconductor Hall device is less than the maximum cross section of the magnetic molecule, and the intervals of the arrayed semiconductor Hall devices are larger than the diameter of the magnetic Hall molecule. Thus, the analysis accuracy is enhanced. An interconnection is used commonly to the semiconductor Hall devices, thereby reducing the size of the sensor.