Provided is a gel particle measurement reagent effective in quickly measuring a time point of initiation of production of gel particles. A gel particle measurement reagent R is a gel particle measurement reagent to be used to be agitated continuously with a sample S containing a target substance St as a measuring object to turn the target substance St into gel particles, including: a reagent base material 1 that undergoes a gelation reaction with the target substance St; and a biologically inactive particle formation accelerating factor 2 that is added to the reagent base material, has solubility in the sample S and dissolves therein at a concentration of 0.002 to 1%, and accelerates production of gel particles G whose particle sizes are centered in a predetermined range.