To provide a new polyphosphonate having a primary hydroxy group in which application as a flame-retardant material and a flame-resistant material can be expected.

The polyphosphonate having a primary hydroxy group is obtained by carrying out polyaddition reaction of a compound having two or more oxetane rings with phosphonic acids in the molecule in the presence of a quaternary onium salt, crown ether complexes or a tertiary amine. In the polyphosphonate, use as the flame-retardant material, the flame-resistant material, etc., can be expected.