The present invention provides an epoxy curing agent comprising colloidal silica particles, which is a liquid having low viscosity and excellent transparency and is suitable as a curing agent for a resin composition for sealing optical semiconductor elements.The epoxy curing agent comprises colloidal silica particles with an average primary particle size of from 5 to 40 nm and a fully saturated dicarboxylic anhydride which is a liquid at 30° C., and it has a light transmittance of at least 60% at a wavelength of 500 nm for a light path length of 10 mm when the concentration of the colloidal silica particles is adjusted to 10 mass %.