The present invention discloses a control gate, including a first exciting coil, a first detection coil, a second exciting coil, a second detection coil, a signal determination unit and an amplification factor setting unit. The first exciting coil forms a first alternating magnetic field in a passage. The first detection coil detects a variation in the first alternating magnetic field. The second exciting coil forms a second alternating magnetic field in the passage. The second detection coil that detects a variation in the second magnetic field. The signal determination unit includes a first amplifier which (i) amplifies a first detection signal of the first detection coil and (ii) determines a passage of the magnetic material based on the amplified first detection signal. The amplification factor setting unit sets a first amplification factor for the first amplifier based on a second detection signal of the second detection coil.