An optical waveguide of a system in which the diameter of a core is constant in the optical wave propagation direction with the diameter of a mode field at input and output ends increased by reducing DELTA n toward the input and output ends, a difference DELTA n between refractive indexes of a core and a clad of the waveguide along the optical wave propagation direction for improving a coupling efficiency and tolerance in a connecting operation having a DELTA n variation region in which DELTA n varies from DELTA n1 to DELTA n2 ( DELTA n1 > DELTA n2), a cross-sectional shape of the core not varying in the optical wave propagation direction, characterized in that an X-Y plot in which the X and Y axes represent the coordinates of DELTA n variation region in the optical wave propagation direction and DELTA n respectively projects downward, and an optical fiber.
