The present paper describes a method to obtain accurate β ±-ray maximum energies with small HPGe detector. In order to unfold β ±-ray spectrum observed, response functions for monoenergetic electrons and positrons were experimentally determined for four different sizes of HPGe detectors with diameters of 16-50 mm and with thicknesses of 10-13 mm. The reliability of the unfolding procedure was confirmed by a comparison between the experimental and its well-evaluated Qβ-value. The systematic error is estimated to be 20 keV in an endpoint energy range of 1-4 MeV, and 30 keV in 4-9 MeV. The present method was applied for the Qβ measurement of the 2 - isomer of 146La; the Qβ-value was obtained to be 6580(60) keV.