In a battery system, battery modules (3a, 3b) connected to each other in series respectively include: one or more single cells (3a1 to 3an, 3b1 to 3b-n) connected to one another in any one of series, parallel, and series-parallel; cell voltage switches (7a, 7b) for detecting voltages respectively of the one or more single cells; module monitoring control units (9a, 9b) each for monitoring the detected voltages respectively of the one or more signal cells; and communications level converter circuits (14a, 14b). The battery system includes a master unit (8a) for receiving information on the voltages respectively of the one or more single cells from the module monitoring control units via the communications level converter circuits. One of the communications level converter circuits includes a switch element (Q32) for transmitting a signal of a low-potential battery module to a high-potential battery module. A series circuit includes resistors R1′, R2′ and R5 as well as the switch element is connected between a positive electrode of the highest-potential single cell in the high potential battery module and a negative electrode of the lowest-potential single cell in the low-potential battery cell.