An optical modulator includes a relay substrate having signal conductor patterns that connect signal input terminals and signal electrodes of an optical modulation element and ground conductor patterns, and a housing, in which the signal conductor pattern includes a component mounting portion including an electrical circuit element. Two ground conductor patterns sandwiching the signal conductor pattern are formed in an asymmetrical shape in a plan view within a component mounting area having a square shape in the plan view centered on the component mounting portion. A direction of a side of the component mounting area having the square shape is same as the extending direction of the signal conductor pattern and a length of a side of the component mounting area is equal to a distance from a center of the component mounting portion to a portion on an adjacent signal conductor pattern closest to the center.