To provide a low-refractive index coating agent which maintains low surface tension, even at high solid concentration and which excels in the leveling properties as a coating solution, in order to acquire a surface which has no unevenness and is superior in smoothness.

The low refractive index coating agent contains an ionization radiation curing type material and a solvent and, further, contains polyether-modified polysiloxane which does not have reactive groups. The change of rate of surface tension (B)((B-A)/A×100), at a solid concentration of 70 wt.% on the basis of surface tension (A) at solid concentration of 3.5 wt.% is within a range between 0% and 7%. Further, a maximum value of a change rate of surface tension (C)((B-C)/B×100) at highest solid concentration, on the basis of surface tension (B) at solid concentration of 70 wt.% lies, within a range between 15% and 30%.