The purpose of the present invention is to provide a PS-QPSK optical modulator that has a simple configuration and no theoretical optical loss. In order to achieve this purpose, the optical modulator is characterized by being provided with an input optical branching means for branching input light from a main input port into two branches, a final optical coupling means for coupling two systems of optical signals in a polarization state where the optical signals are orthogonal to each other and outputting the optical signals to a main output port, an intermediate optical coupling means positioned between the input optical branching means and the final optical coupling means and provided with at least two input ports and at least two output ports, first and second optical paths linking the input optical branching means and the intermediate optical coupling means and having optical path lengths substantially equal to each other, third and fourth optical paths linking the intermediate optical coupling means and the final optical coupling means and having optical path lengths substantially equal to each other, and three binary phase modulation means arranged one each in three of the first, second, third, and fourth optical paths.