An exhaust gas purification catalyst apparatus. The apparatus has a noble metal component for oxidizing NOx in an exhaust gas discharged from a diesel engine, a reducing agent spraying means for supplying the reducing agent selected from a urea component or an ammonia component, and a selective reduction catalyst (SCR) not comprising a noble metal for removing by reduction NOx by contacting with the reducing agent, in this order from the upstream side of an exhaust gas passage. Activity of the selective reduction catalyst (SCR) is maintained by setting that the noble metal component of the oxidation catalyst (DOC) comprises platinum and palladium, and ratio of platinum particles existing alone is 20% or less, or average particle diameter of the noble metal is 4 nm or larger, and by suppressing volatilization of platinum from the oxidation catalyst (DOC), even when catalyst bed temperature increases up to 900° C.