A partial block polyimide-polysiloxane copolymer is provided comprising repeat unit structures having structural formulae (1) and (2) wherein X is an aromatic or aliphatic ring-containing tetravalent organic radical, Y1 is a diamine residue, Y2 is a diaminosiloxane residue, Y1 and Y2 are contained in the copolymer in amounts of 99-20 mol % and 1-80 mol %, respectively, L and m each are an integer of 2-50. The copolymer has good adhesion to substrates, moisture-proof reliability and a low modulus of elasticity.