A transmission type optical deflector wherein a prism holder is supported by a yoke member to rotate about a principal axis through an elastic device. A wedge-shaped transmission prism is provided on the prism holder to transmit and refract light. The prism holder and yoke member are respectively provided with a coil and a permanent magnet secured thereto, to produce a rotational motion of the prism holder in the forward and reverse directions about the principal axis due to an electromagnetic function. The transmission prism is partly cut away at the thick portion thereof so that the center of gravity of the transmission prism is identical to the principal axis in a section in which the principal axis appears as a point and the transmission prism appears as a wedge-shape.