Resin sheets which includes a support and a resin composition layer in contact on the support, and which are characterized in that an extracted water conductivity A of a cured product of the resin composition layer when extracted at 120° C. for 20 hours is 50 μS/cm or less and an extracted water conductivity B of the cured product of the resin composition layer when extracted at 160° C. for 20 hours is 200 μS/cm or less, can provide a thin insulating layer having excellent insulating properties.