[en] A method to determine the β⁺-ray maximum energy with a high purity germanium (HPGe) detector is proposed. The response function of an HPGe detector was determined by measurements of monoenergetic positrons with energy up to 3.8 MeV. The energy dependence of the response function was described in analytical functions including 14 parameters. Using the response function, β⁺-ray maximum energies for ⁶²Cu and ⁶⁸Ga with well-known Q_EC-values were measured. The accuracy of β⁺-ray maximum energy determination in the present method is estimated to be better than 20 keV in good statistics. The method was applied to ⁹¹Mo (T_1/2=15.49 min); its Q_EC-value was measured to be 4.435(23) MeV. (orig.)