Abstract: A curable composition comprises an insulating resin and a halogen-free flame retardant. The halogen-free flame retardant has a particulate form, and whose primary particles have an average major axis from 0.01 to 5 ?m, an aspect ratio of 5 or less, and the proportion of a major axis of more than 10 ?m being at most 10% by number. A varnish comprises an insulating resin, a curing agent, a flame retardant and an organic solvent. The flame retardant is a flame retardant in particulate form surface-treated with a coupling agent, and the flame retardant particles present in the varnish have a secondary particle diameter of 30 ?m or less.