There is provided a planar antenna device having a first antenna and a second antenna arranged on a front surface of a substrate to be set in parallel to an up and down direction. The first antenna and the second antenna are arranged such that the magnitude of the difference between the lengths of predetermined sections of the transmission line parts becomes one of a positive odd multiple of a half wavelength of the guide wavelength of electric waves which can be transmitted in the transmission line parts and a positive even multiple of a half wavelength of the guide wavelength. The predetermined sections are provided in sections from the connection parts to the antenna elements.