A resin composition of the present invention is a resin composition used for forming a stress relaxation layer (102) of a metal base copper-clad laminate (100) configured by laminating a metal plate (101), the stress relaxation layer (102), and a piece of copper foil (103) in this order, the resin composition including: an epoxy resin having a polyether structure; a phenoxy resin; and a heat dissipation filler, in which the resin composition satisfies a characteristic of a storage elastic modulus at 25° C. being equal to or more than 0.01 GPa and equal to or less than 1.6 GPa.