In a battery production process, a positive electrode active material having a reaction-suppressing layer that does not easily peel off formed on the surface thereof, and a positive electrode and an all-solid-state battery that use said material are provided. The present invention involves positive electrode active material particles for an all-solid-state battery containing sulfide-based solid electrolyte. The positive electrode active material particles are an aggregate containing two or more particles. The surface of the aggregate is coated with a reaction-suppressing layer for suppressing reactions with the sulfide-based solid electrolyte.