An apparatus and method for detecting total voltage of an assembled battery including a number of serially-connected electric cells. Voltages of the respective cells are detected and added up, correlation between the resulting total voltage and a total voltage obtained by detecting the voltage of the assembled battery directly from its terminals is obtained, and a corrective arithmetic operation is carried out inside the device to correct the total voltage obtained through direct detection. The total voltage is detected with a high degree of accuracy without the need for individual adjustment operations using a variable resistance, high-accuracy external tester.