The purpose of the present invention is to improve the efficiency of conversion between terahertz electromagnetic wave energy and direct current energy via plasma waves in a terahertz electromagnetic wave conversion device with a field effect transistor structure. This terahertz electromagnetic wave conversion device has an HEMT structure having a substrate (201), an electron transit layer (202), an electron supply layer (203), a source (204) and a drain (205), and comprises a first group of gates (G1) and a second group of gates (G2). The gate length (L1) of each finger of the first group of gates (G1) is narrower than the gate length (L2) of each finger of the second group of gates (G2), and each finger of each group of gates is disposed between the source (204) and the drain (205) on the same cycle (W). A first distance (D1) and a second distance (D2) from each finger of the first group of gates (G1) to two fingers of the second group of gates (G2) adjoining that finger are set to unequal lengths
