The present invention provides an optical system with waveguides, which comprises first, second and third optical input/output means (12, 14, 16), fourth and fifth multi-mode optical waveguides (20, 22) each capable of propagating light with plural propagation modes, and optical-filter mounting means (26) for mounting an optical filter (24) between the fourth and fifth multi-mode optical waveguides (20, 22) across a traveling direction of light in the fourth and fifth multi-mode optical waveguides (20, 22). The first optical input/output means (12) is connected to an end face of the fourth multi-mode optical waveguide (20) on a side thereof opposite to the optical-filter mounting means (26). Each of the second and third optical input/output means (14, 16) is connected to an end face of the fifth multi-mode optical waveguide (22) on a side opposite to the optical-filter mounting means (26). Respective widths in a direction orthogonal to the light-traveling direction of the fourth and fifth multi-mode optical waveguides (20, 22) are different from each other.