[en] We report on the experimental results of the ionization efficiency of Rb and Cs using a surface ionization source prototype at Rare Isotope Science Project (RISP) of Institute for Basic Science (IBS). A hot-cavity surface ionization source has been studied in an off-line test bench equipped with a mass separator and a beam diagnostic system which allow accurate ionization efficiency measurements. Ionization efficiencies of 76% and 77% have been measured for Rb and Cs, respectively, from the Ta prototype of the RISP surface ionization source.