An optical device includes a rectangular parallelepiped prism including a reflection-transmission surface for reflecting and transmitting light fluxes, a seating surface provided so that a bottom surface of the prism is fixed by an adhesive, and a groove portion provided in a part of the periphery of the seating surface. When the prism is fixed to the seating surface by the adhesive, the groove portion is configured to be capable of receiving the adhesive protruded from between the bottom surface of the prism and the seating surface when the prism is pressed against a first positioning member for determining a position of the prism so that a first side face of the prism is along a predetermined straight line and against a second positioning member for restricting a second side face orthogonal to the first side face of the prism from moving in the direction of the straight line.