An oscillation element that oscillates an electromagnetic wave includes a negative resistance element and a resonator including a first conductor and a second conductor, in which the negative resistance element and the resonator are arranged on a substrate, the negative resistance element is electrically connected to the first conductor and the second conductor, the first conductor and the second conductor are capacitively coupled to each other, and when a capacitance between the first conductor and the second conductor is set as C, an inductance of the first conductor and the second conductor is set as L.sub.1, a speed of the oscillated electromagnetic wave in vacuum is set as C.sub.0, a relative dielectric constant of the substrate is set as .epsilon..sub.r, and a diagonal line length of the substrate is set as d, a series resonant frequency f.sub.1 of the resonator satisfies f.sub.1=1 / {2.pi. (L.sub.1C)}, and f.sub.1<C.sub.0 / [d {(1+.epsilon..sub.r) / 2}].