Provided are: an insulating film which has excellent production cost and is capable of ensuring sufficient adhesive force; and a flat cable. The present invention is an insulating film which is obtained by sequentially laminating a resin film, a primer layer containing a resin component, and an adhesive layer in this order, and wherein the primer layer contains a glycidyl group-containing polymer such that the content of the glycidyl group-containing polymer is 80% by mass or more of the resin component. The glycidyl group-containing polymer is preferably a thermoplastic resin. It is preferable that the primer layer additionally contains a polyolefin resin. It is preferable that the primer layer additionally contains a filler and the content of the filler is from 10 parts by mass to 100 parts by mass (inclusive) relative to 100 parts by mass of the resin component. The main component of the adhesive layer is preferably a polyolefin resin. The adhesive layer preferably contains a flame retardant. It is preferable that the adhesive layer is composed of a plurality of layers, which include an outer layer and an inner layer that is arranged between the outer layer and the primer layer, and the flame retardant is contained in the inner layer.