A gel particle measurement device for measuring particles produced from a target substance in a sample through a gelation reaction, the gel particle measurement device comprising: (a) a sample cuvet (1, 100) at least partially comprising an incident portion through which light enters, and accommodating a sample containing a target substance to be measured and a solution containing a reagent that causes gelation of the target substance; (b) stirring means (2, 120, 121) for stirring a mixed solution comprising the sample and the solution containing the reagent in the sample cuvet so as to inhibit gelation of the entire mixed solution; (c) an incident light source (3, 130) provided outside the incident portion of the sample cuvet, for irradiating the mixed solution comprising the sample and the solution containing the reagent in the sample cuvet with coherent light; (d) backscattered light detecting means (4, 140) provided outside the incident portion of the sample cuvet on the same side on which the incident light source is provided, the backscattered light detecting means detecting a backscattered light component, which returns toward the incident light source, in the light scattered in the mixed solution comprising the sample and the solution containing the reagent in the sample cuvet; (e) scattered light fluctuation measuring means for measuring a fluctuation component of scattered light based on a detection output from the backscattered light detecting means; (f) stray light removing means (8, 150) provided on the opposite side on which the incident light source and the backscattered light detecting means are provided in the sample cuvet or around the sample cuvet, the stray light removing means being configured for removing a stray light component, which is generated by transmission or scattering, except the backscattered light component, which returns toward the incident light source in the mixed solution, in irradiation light from the incident light source; and (g) gel particle production determining means (6, 160) for determining, based on a result of measurement by the scattered light fluctuation measuring means, a production state of gel particles, which includes at least a starting point of production of the gel particles in the mixed solution, which leads to timing of phase transition of the mixed solution from a sol phase to a gel phase. (a) a sample cuvet (1, 100) at least partially comprising an incident portion through which light enters, and accommodating a sample containing a target substance to be measured and a solution containing a reagent that causes gelation of the target substance; (b) stirring means (2, 120, 121) for stirring a mixed solution comprising the sample and the solution containing the reagent in the sample cuvet so as to inhibit gelation of the entire mixed solution; (c) an incident light source (3, 130) provided outside the incident portion of the sample cuvet, for irradiating the mixed solution comprising the sample and the solution containing the reagent in the sample cuvet with coherent light; (d) backscattered light detecting means (4, 140) provided outside the incident portion of the sample cuvet on the same side on which the incident light source is provided, the backscattered light detecting means detecting a backscattered light component, which returns toward the incident light source, in the light scattered in the mixed solution comprising the sample and the solution containing the reagent in the sample cuvet; (e) scattered light fluctuation measuring means for measuring a fluctuation component of scattered light based on a detection output from the backscattered light detecting means; (f) stray light removing means (8, 150) provided on the opposite side on which the incident light source and the backscattered light detecting means are provided in the sample cuvet or around the sample cuvet, the stray light removing means being configured for removing a stray light component, which is generated by transmission or scattering, except the backscattered light component, which returns toward the incident light source in the mixed solution, in irradiation light from the incident light source; and (g) gel particle production determining means (6, 160) for determining, based on a result of measurement by the scattered light fluctuation measuring means, a production state of gel particles, which includes at least a starting point of production of the gel particles in the mixed solution, which leads to timing of phase transition of the mixed solution from a sol phase to a gel phase.