Brake pad assembly and caliper

This brake pad assembly is, for example, equipped with: a brake pad that includes a lining and a back plate; a detection unit attached to the plate and detects abrasion of the lining; and a retainer for attaching the unit to the plate in a detachable manner. The retainer has: a base part that contacts a surface of the plate; a first holding part that holds the retainer in a prescribed position on the plate; a second holding part provided so as to be movable between a restricting position in which the unit is restricted from separating from the plate, and a release position in which the unit can be detached from the plate; a biasing part that elastically biases the second holding part toward the restricting position; and an operation part that moves the second holding part to the release position by means of an applied external force.