A cylinder block plate processing apparatus in which one end side of an inner peripheral surface of a cylinder of a cylinder block is sealed to circulate a process liquid to perform pre-plating or plating processing of the cylinder inner peripheral surface. The apparatus includes: an apparatus body having a workpiece mount table on which the cylinder block is placed; a workpiece holding tool that is provided on the workpiece mount table so as to be vertically movable; an electrode support member including an electrode cylinder mounted to the apparatus body; a processing solution supply member that supplies a processing solution to the electrode support member; an electrode operated by the electrode cylinder; a sealing jig provided to one end of the electrode; a driving mechanism that drives the sealing jig; and a control circuit that controls operations of the processing solution supply member and the driving mechanism. The electrode and the sealing jig are inserted from the other end side of the cylinder inner peripheral surface into the cylinder, and the driving mechanism is inserted from an end side of the inner peripheral surface so as to be connectable to the sealing jig.