The deposition of Pd black on the surface of a Pd sheet electrode was carried out chemically using active hydrogen passing through the electrode, and the Pd black deposits were found to grow up three-dimensionally. The hydrogenation rate of 4-methylstyrene increased over 40 times by using the palladinized Pd sheet electrode. The increase in catalytic activity is thought to be due to the increase in reaction zone and appearance of new active sites.