Abstract: This invention relates to a rotary head assembly in which a magnetic tape runs while being would helically, consisting of a fixed cylinder 21 having a fixed cylindrical part, and a rotatable rotary member 19 having a diameter approximately equal to that of said fixed cylinder 21, being disposed coaxially with a very small gap and holding a magnetic head 29, wherein pump-out type spiral grooves 20 are provided either partly or wholly on either plane of the confronting planes formed in the relative rotating part between said fixed cylinder 21 and said rotary member 19 in order to create a buoyancy on a running magnetic tape by air pressure not only in the rotary cylinder area but also in the fixed cylinder area, to realize a stable and smooth tape running, and to radically eliminate the tape chirping phenomenon, sticking phenomenon, and tape damage and cylinder wear accompanying sliding.