The catalyst disclosed is a catalyst including palladium-ruthenium alloy fine particles in which palladium and ruthenium form a solid solution. The palladium-ruthenium alloy fine particles used in this catalyst can be produced by a production method including the step of maintaining a solution containing a protective agent, a reducing agent, a palladium compound or palladium ions, and a ruthenium compound or ruthenium ions at a temperature equal to or higher than a predetermined temperature.