Silica-containing insulating composition

An insulating composition comprising silica particles (A), a resin (B), and a curing agent (C), wherein:

• the silica particles (A) have a property that when an aqueous sol formed from the silica particles (A), excluding the resin (B) and the curing agent (C), having an SiO2 concentration of 3.8% by mass, is heated at 121° C. for 20 hours, an amount of Na ions eluted from the silica particles (A) is 40 ppm/SiO2 or less;
• the silica particles (A) contain a polyvalent metal oxide such that a ratio by mole of a polyvalent metal M to Si is 0.001 to 0.02;
• a mass ratio of Na2O/SiO2 in the silica particles (A) is 700 ppm to 1,300 ppm, but wherein surface regions, having a thickness of from 0.1 nm to 1.5 nm as measured from the surface of the silica particles (A), have a Na2O/SiO2 mass ratio of 10 ppm to 400 ppm;
• and
• the silica particles (A) have an average particle diameter of from 5 nm to 40 nm.