Magnetization of MnAs_1−xSb_x was measured as functions of temperature and magnetic field for 0≤x≤0.4. The entropy change caused by a magnetic field, ΔS_mag, was estimated on the basis of the Maxwell relation. The ΔS_mag for 0≤x≤0.3 in a field change of 5 T reaches 25–30 J/K kg, which exceeds that of other materials by a factor of 2–4. The substitution of Sb for As can tune the Curie temperature between 230 K and 315 K without any significant reduction of ΔS_mag. The large ΔS_mag originates in a paramagnetic to ferromagnetic transition induced by a magnetic field. These results indicate that MnAs_1−xSb_x is a promising material for a working substance in magnetic refrigeration near room temperature.