An object of the present invention is to provide a practical solid polymer electrolyte that exhibits excellent thermal characteristics, physical characteristics, and ionic conductivity, and in particular, to provide a completely solid electrolyte and a copolymer composition that functions as the substrate for producing such an electrolyte. A solid polymer electrolyte comprising an electrolyte salt, and a copolymer in which a block chain A containing a repeating unit represented by a formula (I) shown below, ##STR00001## a block chain B containing a repeating unit represented by a formula (II) shown below: ##STR00002## (wherein, R.sub.9 represents an aryl group), and a block chain C containing a repeating unit represented by a formula (III) shown below: ##STR00003## (wherein, R.sub.13 represents an aryl group or a heteroaryl group) are arranged in the sequence B, A, C.