There is disclosed resin composition comprising 1 to 99% by weight of a polymer A and 1 to 99% by weight of a polymer B, the polymer A and B having refractive indexes which permit satisfaction of the relation:a.ltoreq.0.015 and b-a.gtoreq.0.005wherein a and b are the minimum and the maximum, respectively, of absolute value of the difference between the refractive indexes of the polymers A and B in the temperature range of 20.degree. to 100.degree. C.; said composition being useful as a functional resin for buildings, etc.