It is an object of the invention to provide an optical modulator in which a connection substrate or a terminal substrate is disposed outside an optical modulation element and which can maintain at a proper voltage amplitude value a modulation signal applied to an optical modulation element. An optical modulator includes: a substrate having electro-optic effect; an optical waveguide formed on the substrate; an optical modulation element 1 having a modulation electrode (which includes a signal electrode 2) for modulating light passing through the optical waveguide; and a connection substrate 20 disposed outside the substrate to supply a modulation signal for driving the optical modulation element to the optical modulation element. In the optical modulator, a signal line for propagating the modulation signal is formed on the connection substrate, and a modulation signal monitoring output line 25 for monitoring the voltage amplitude of the modulation signal is provided right in front of an end of the signal line for supplying the modulation signal from the signal line to the optical modulation element.