Abstract: To provide a novel technique with which the generation of aggregates over time in a dispersion liquid can be suppressed and a target substance can be detected with high sensitivity even if latex particles that have been stored are used. A method for storing in a container a latex particle dispersion liquid in which latex particles for an extracorporeal diagnostic agent are dispersed, the latex particles having a volume average particle size of 10 to 1000 nm and including a polymer containing 70 to 100% by mass of monomer units each derived from a monomer having an aryl group relative to the total monomer units, which is characterized in that the particle dispersion liquid is stored in the container by setting a ratio of a volume of a void space obtained by excluding a volume occupied by the particle dispersion liquid from an internal volume of the container to 0 to 25% (v/v) relative to the internal volume of the container.