Electromagnetic wave oscillators each having a multi-tunnel and electromagnetic wave generating apparatuses including the electromagnetic wave oscillators are provided. The electromagnetic wave oscillator includes: a first waveguide which has a folded structure such that a path traveled by an electromagnetic wave through the first waveguide crosses an axial direction a plurality of times; an electron beam tunnel through which an electron beam passes, wherein the electron beam tunnel extends along the axial direction and crosses the first waveguide a plurality of times; and at least one auxiliary tunnel which extends parallel to the electron beam tunnel and which crosses the first waveguide a plurality of times.