Crystalline polyethernitrile

A crystalline polyethernitrile with a difference between a melting point and a crystallization temperature at a time of temperature-fall being 40° C. or more and 100° C. or less, the crystalline polyethernitrile comprising N repeating units represented by Formula (I) and M repeating units represented by Formula (II), N and M being integers satisfying 0.90<[N/(N+M)]<1.00:

• wherein Formulas (I) and (II) satisfy (A) or (B):
• (A) Ar1 has one framework selected from units represented by Formula (e) and Formula (f), and Ar2 has one framework selected from units represented by Formula (a) to Formula (c), Formula (e), and Formula (f), with a proviso that Ar1 and Ar2 are not the same;
• and
• (B) Ar1 has one framework selected from units represented by Formula (a) to Formula (c), and Ar2 has one framework selected from units represented by Formula (c) and Formula (e), with proviso that Ar1 and Ar2 are not the same,
• in Formulas (I) and (II) and Formula (a) to Formula (c), Formula (e), and Formula (f), Ra is any one of a linear organic group, a branched organic group, or a cyclic organic group having a carbon number of 1 to 6, and optionally has one or more oxygen atoms, nitrogen atoms, and sulfur atoms;
• Ra is same as or different from each other;
• a represents number of substituents in Ra and is an integer from 0 to 4;
• and X is a hydrogen atom or a methyl group.