PURPOSE: To suppress unnecessary oscillation, by locating an electromagnetic wave absorbing body to a space excluding the direction toward noise output terminal out of the surrounding space of an IMPATT diode.

CONSTITUTION: An IMPATT diode 2 is fixed in a waveguide 1 having a noise output terminal via a bias applying conductor 4. Electromagnetic wave absorbing bodies 5, 6 are located around the conductor 4 and inside the end of the waveguide 1. The electromagnetic wave absorbing bodies are located by taking iron carbonyl cured with resin as material so that they occupy the surrounding space excluding the direction toward the noise output terminal of the diode 2. With the constitution like this, since Q is lowered over the entire area of frequency band by the bodies 5, 6, spurious oscillation can be suppressed in the spurious resonance circuit formed around the diode 2, and noise output can be obtained in broad band.