To obtain a loop antenna which can receive a fine radio wave and is suitably used for an authentication device at low cost through easy manufacture by forming subordinate loop antennas of one conductor nearby a main loop antenna, so that a magnetic field is excited in a direction opposite from the main loop antenna.

A main loop antenna 1 is provided in proximity with four subordinate loop antennas 2 to 5, and the overall area of the subordinate loop antennas is equal to the area of the main loop antenna 1, and the excited magnetic field is opposite in phase from that of the main loop antenna 1. Both the loop antennas 1 to 5 are formed of one conductor, which exits from a couple of feeding terminals 6 and returns to them in one stroke on both the surfaces of a printed board 7, so that they do not cross one another. Consequently, magnetic fields are in the same direction near the main loop antenna 1 and subordinate loop antennas 3 and 5 and magnetic fluxes are overlapped to increase in electric field intensity, and magnetic fields at distant positions become opposite in directions to decrease in intensity. The subordinate loop antennas 2 and 4 also operate similarly.