An oscillator using spin transfer torque includes i) a pinned magnetic layer having a fixed magnetization direction, ii) a non-magnetic layer located on the pinned magnetic layer, and iii) a free magnetic layer located on the non-magnetic layer. The pinned magnetic layer includes i) a first part of the fixed magnetic layer and ii) a second part of the fixed magnetic layer located thereon. The first part of the fixed magnetic layer includes i) a first interface in contact with the second part of the fixed magnetic layer and ii) a second surface exposed to an outside while surrounding the first interface.