Optical modulator

An optical modulator including an optical modulation element mounted inside a housing, in which at least a portion of an electric line is connected to an external circuit board through a flexible printed circuit, and which is disposed on the external circuit board,

• wherein a recess portion that mounts the flexible printed circuit is formed on an outer bottom surface of the housing and at a part where the flexible printed circuit is connected,
• a plurality of signal lines disposed in the flexible printed circuit,
• means for preventing a resonance mode are provided such that a resonance mode of a microwave and a millimeter wave or a parallel plate mode is prevented from being generated among a surface A of the recess portion facing the flexible printed circuit, an electric wiring part B provided on the flexible printed circuit overlapping the surface A when seen in a plan view, and the external circuit board overlapping the surface A when seen in a plan view, and
• wherein in the means for preventing a resonance mode, no conductor including an electric wiring part is provided on a surface of the external circuit board overlapping the surface A when seen in a plan view and at least one of a clearance or a dielectric constant between the surface A and the electric wiring part B is set to be locally varied.