Provided is a loop antenna which can contribute to an increase of an area of a radio system using a magnetic field. The loop antenna includes a main loop 1 which is an open loop connected to a signal source 5 or a reception circuit; and an amplification loop 2 which is a closed loop having the same shape as the main loop 1. The main loop 1 and the amplification loop 2 are arranged on a same surface of a flat substrate formed of an insulator. A first capacitance is connected to the main loop 1, and a second capacitance is connected to the amplification loop.