The present invention provides a process of solidifying a phosphoric ester-based flame retardant in which high-level purification is not required and stress load is not employed.In the process of solidifying a phosphoric ester-based flame retardant, a seed crystal is added at a temperature of not higher than 55° C. and at an amount of 1 to 50 parts by mass with respect to 100 parts by mass of a phosphoric ester compound in a molten state, which compound is represented by the following Formula (I): (wherein, R1 and R2 each independently represent a hydrogen atom or a methyl group; and n represents a number of 1 to 5). It is preferred that the amount of the phosphoric ester compound represented by the aforementioned Formula (I) wherein n is 1 be less than 90% by mass and that R1 and R2 be a hydrogen atom.