To provide a test reagent composed of dispersion liquid in which dispersoid is less likely to separate even when centrifugal force is applied to a microchip.

The test reagent of the present invention is used for a microchip for measuring an object to be measured. The test reagent is composed of dispersion liquid including a composite particle having a core particle and a metal layer covering a surface of the core particle, as dispersoid. Specific gravity of the composite particle is 10 or less. The core particle is preferably composed of material having the specific gravity of 0.8-1.2. The core particle is preferably composed of resin. In addition, the dispersion liquid is preferably colloidal dispersion.