The rotary transformer 5 includes a plate-shaped stator 51 having a coil arranged on one surface thereof, and a plate-shaped rotator 52 having a coil arranged on one surface thereof, in which the respective coil-arranged surfaces are disposed to face with each other so that signal transmission in a frequency range of 1 to 10 MHz is performed between the coils. The stator includes a substrate 512 on which a plurality of one-turn coils are formed into a concentric shape, and a supporting member 513. The rotator includes a substrate 522 on which one-turn coils 521 of same number are formed into a concentric shape, and a supporting member 523. Air or a material M which is an insulator and has a relative magnetic permeability substantially equal to 1 is lying between the substrate 512 and the supporting member 513, and between the substrate 522 and the supporting member 523.