An optical device comprising: a rectangular parallelepiped prism (24a, 24b) configured to include a reflection-transmission surface (25a, 25b) for reflecting and transmitting light fluxes; a single seating surface, formed on a substrate, provided so that a bottom surface of the prism (24a, 24b) is fixed to the seating surface by an adhesive; a groove portion provided in at least a part of the periphery of the seating surface; openings provided in the substrate at positions opposite to the seating surface across the groove portion for receiving a first positioning member (27, 27a, 40) for positioning the prism (24a, 24b) on the seating surface such that a first side face of the prism is along a predetermined straight line, and a second positioning member (28, 42) for restricting a second side face of the prism orthogonal to the first side face from moving in a direction of the straight line, and a surface filling a space between the openings for receiving the first positioning member (27, 27a, 40) and the second positioning member (28, 42) and the groove portion, the height of the surface being the same as or lower than the height of the seating surface, and higher than the groove portion, wherein the groove portion is formed so as to be capable of receiving the adhesive protruded from between the bottom surface of the prism (24a, 24b) and the seating surface when the prism (24a, 24b) is pressed against the first positioning member (27, 27a, 40), for determining a position of the prism (24a, 24b) so that the first side face of the prism (24a, 24b) is along the predetermined straight line, and against the second positioning member (28, 42), for restricting the second side face orthogonal to the first side face of the prism (24a, 24b) from moving in the direction of the straight line, at the time of fixing the prism (24a, 24b) to the seating surface by the adhesive, such that excess adhesive does not reach the first positioning member (27, 27a, 40) and the second positioning member (28, 42). a rectangular parallelepiped prism (24a, 24b) configured to include a reflection-transmission surface (25a, 25b) for reflecting and transmitting light fluxes; a single seating surface, formed on a substrate, provided so that a bottom surface of the prism (24a, 24b) is fixed to the seating surface by an adhesive; a groove portion provided in at least a part of the periphery of the seating surface; openings provided in the substrate at positions opposite to the seating surface across the groove portion for receiving a first positioning member (27, 27a, 40) for positioning the prism (24a, 24b) on the seating surface such that a first side face of the prism is along a predetermined straight line, and a second positioning member (28, 42) for restricting a second side face of the prism orthogonal to the first side face from moving in a direction of the straight line, and a surface filling a space between the openings for receiving the first positioning member (27, 27a, 40) and the second positioning member (28, 42) and the groove portion, the height of the surface being the same as or lower than the height of the seating surface, and higher than the groove portion, wherein the groove portion is formed so as to be capable of receiving the adhesive protruded from between the bottom surface of the prism (24a, 24b) and the seating surface when the prism (24a, 24b) is pressed against the first positioning member (27, 27a, 40), for determining a position of the prism (24a, 24b) so that the first side face of the prism (24a, 24b) is along the predetermined straight line, and against the second positioning member (28, 42), for restricting the second side face orthogonal to the first side face of the prism (24a, 24b) from moving in the direction of the straight line, at the time of fixing the prism (24a, 24b) to the seating surface by the adhesive, such that excess adhesive does not reach the first positioning member (27, 27a, 40) and the second positioning member (28, 42).