A resin optical waveguide containing a core, an under cladding and an over cladding having refractive indices lower than that of the core, in which the resin optical waveguide has, at one end side thereof, a core-exposed section at which the over cladding is not present and the core is exposed and, of the under cladding, a portion corresponding to the core-exposed section has a core-neighboring region that satisfies the following (1) and (2):(1) the core-neighboring region is a region whose distance from the core is within x, and x is 5 μm or more and 20 μm or less; and(2) the core-neighboring region has a refractive index distribution that the refractive index at a side of an interface with the core is high and the refractive index at a far side from the interface with the core is low.