A helical antenna includes a ground plate, a first helical portion spirally wound perpendicular to the plate, a second helical portion spirally wound perpendicular to the plate and surrounding the first helical portion radially outward of the first helical portion, and a feeder circuit. The circuit includes an oscillator, a divider connected to the oscillator, a first phase shifter connected between a first output terminal of the divider and a feeding point of the first helical portion, and a second phase shifter connected between a second output terminal of the divider and a feeding point of the second helical portion. Length of one turn of the first helical portion is equal to a result of multiplication of a wavelength of oscillation of the oscillator by N. Length of one turn of the second helical portion is equal to a result of multiplication of the wavelength by M (M>N).