We have developed a compact retarding-type Mott spin polarimeter with a large collection angle at the Hiroshima Synchrotron Radiation Center and obtained a reliable value of the effective Sherman function by a self-calibration method. The spin polarization of secondary electrons was evaluated by extrapolation of the left-right scattering asymmetry at the zero energy-loss limit, which enabled us to obtain the effective Sherman function (Seff) and the figure of merit of our polarimeter at different operating conditions.