An optical modulator includes an optical modulation substrate, an electrical length adjusting substrate, a package containing the substrates, and a plurality of input ports for inputting high frequency electrical signals. The optical modulation substrate includes a substrate body made of an electro-optic material, a ground electrode and a plurality of signal electrodes provided on the substrate body, optical waveguides propagating lights interacting with the signal electrodes, respectively, and electrode input ports inputting the high frequency electrical signals into the signal electrodes, respectively. The signal electrode includes an interacting part, an input end part provided between the electrode input port and interacting part, and a terminal part. The electrical length adjusting substrate includes conductive lines connected to the input ports for inputting the high frequency electrical signals, respectively. The conductive lines have electrical lengths different from each other for adjusting the phase differences among the ports.