A planar antenna including slot arrays configured to set a narrow interval between elements so as to perform beam scanning in a wide angle range while keeping low loss and low profile. The planar antenna includes: a coaxial line including an inner conductor, an outer conductor provided so as to surround a circumference of the inner conductor, and both ends short-circuited; a feeding mechanism for exciting the coaxial line; and a plurality of slots formed on the outer conductor with a certain angle with respect to a tube direction of the coaxial line and having approximately a resonance length.