To provide latent hardening agent particles high in storing stability and also high in hardening property.

In the invention, the content of a main hardening agent of the latent hardening agent particle is increased by impregnating resin particles with the main hardening agent, and an adhesive is hardened at a low temperature. The latent hardening agent particles having undecomposed main hardening agent can be prepared by impregnating the resin particles with the main hardening agent after manufacturing of the resin particles even if the main hardening agent is such an agent decomposing in a manufacturing process. The latent hardening agent particles containing both of an auxiliary hardening agent and the main hardening agent can be prepared by making an impregnating liquid contain the auxiliary hardening agent. In the latent hardening agent particle like this, the adhesive can be hardened even when the auxiliary hardening agent is not added into a binder of the adhesive.