Provided is a resin optical waveguide containing a core, under cladding and over cladding, in which the resin optical waveguide has portions having a core width varying along a light propagation direction, the maximum core width is 4 to 10 μm, and the minimum core width of 1 μm or more and less than 4 μm, when the length of a portion S at which the core width is 1 μm or more and less than 4 μm is LS and the length of a portion at which the core width is 4 to 10 μm is LL, the proportion of LS to the total length is 0.1 to 40%, and the portion S contains neither a certain bubble defect nor a certain defect inside the core and in a vicinity of a core-cladding interface.