A Pd sheet electrode was chemically palladinized with active hydrogen passing through it. The Pd black deposits grew up three-dimensionally with increasing palladinization time, causing the marked increase in reaction zone or number of reaction site for the hydrogenation. The rate of 4-ethyltoluene production (v) in the hydrogenation of 4-methylstyrene at the palladinized Pd sheet electrodes increased over 40 times larger than that : at the Pd sheet electrode without palladinization. In addition, the reciprocal of the v value linearly depended on the reciprocal of surface roughness factor, supporting the idea that the rate-determining step changed from the hydrogenation to the supply of the active hydrogen together with the increase in surface roughness factor of the palladinized Pd sheet electrode.