This invention is a resin composition for optical wiring, comprising an inorganic filler with an average particle size of 1 nm to 100 nm and a resin, having a ratio n.sub.f/n.sub.r (where n.sub.f is the refractive index of the inorganic filler and n.sub.r is the refractive index of the resin) of 0.8 to 1.2, a thermal expansion coefficient of -1.times.10.sup.-5 /.degree. C. to 4.times.10.sup.-5/.degree. C., and a true dependency value of its refractive index on the temperature of -1.times.10.sup.-4/.degree. C. to 1.times.10.sup.-4/.degree. C. in a temperature range from -20.degree. C. to 90.degree. C., and substantially incapable of absorbing light in a wavelength range from 0.6 to 0.9 .mu.m or from 1.2 to 1.6 .mu.m. This invention also provides an optoelectronic circuit board.