Crystalline polymer exhibiting solid phase transition phenomenon and use thereof

A crystalline polymer exhibiting crystal transition in the solid phase state, which satisfies the relationship defined by the formula 150>ΔHtr>1.6Ttr−3.5 (1) (wherein ΔHtr represents the endotherm (J/g) accompanying crystal transition and Ttr represents the crystal transition temperature (° C.)). This crystalline polymer has a weight average molecular weight of 600 thousand or less and a crystal transition temperature (Ttr) of 67° C. or below. Since this crystalline polymer has a low phase transition temperature, a high heat of a crystal transition, and a high melting point, the potential utility thereof as a switching element or a thermal storage material used at around normal environment temperatures (20 to 50° C.) is high.