A secondary battery system comprising: two or more modules including a large number of battery cells as secondary batteries and a casing containing the battery cells; a monitoring unit configured to identify a module having leakage of at least active material, among the two or more modules; a main gas pipe provided between the two or more modules and the monitoring unit; a plurality of auxiliary gas pipes provided in correspondence with the modules, at least one of the auxiliary gas pipes being attached to each of the modules; a plurality of solenoid valves provided in correspondence with the auxiliary gas pipes, the solenoid valves being configured to be opened/closed for connection/disconnection between the corresponding auxiliary gas pipes and the main gas pipe, wherein the monitoring unit includes: a pump connected to the main gas pipe and configured to suck a gas supplied into the main gas pipe from the auxiliary gas pipe, into the monitoring unit; a SO 2 gas detection sensor configured to detect a SO 2 gas flowing through the main gas pipe; a failure module identification unit configured to identify a module having leakage of active material based on an output from the SO 2 gas detection sensor; and a sequence controller configured to perform opening/closing operation of the plurality of solenoid valves in accordance with a predetermined sequence. two or more modules including a large number of battery cells as secondary batteries and a casing containing the battery cells; a monitoring unit configured to identify a module having leakage of at least active material, among the two or more modules; a main gas pipe provided between the two or more modules and the monitoring unit; a plurality of auxiliary gas pipes provided in correspondence with the modules, at least one of the auxiliary gas pipes being attached to each of the modules; a plurality of solenoid valves provided in correspondence with the auxiliary gas pipes, the solenoid valves being configured to be opened/closed for connection/disconnection between the corresponding auxiliary gas pipes and the main gas pipe, wherein the monitoring unit includes: a pump connected to the main gas pipe and configured to suck a gas supplied into the main gas pipe from the auxiliary gas pipe, into the monitoring unit; a SO 2 gas detection sensor configured to detect a SO 2 gas flowing through the main gas pipe; a failure module identification unit configured to identify a module having leakage of active material based on an output from the SO 2 gas detection sensor; and a sequence controller configured to perform opening/closing operation of the plurality of solenoid valves in accordance with a predetermined sequence. a pump connected to the main gas pipe and configured to suck a gas supplied into the main gas pipe from the auxiliary gas pipe, into the monitoring unit; a SO 2 gas detection sensor configured to detect a SO 2 gas flowing through the main gas pipe; a failure module identification unit configured to identify a module having leakage of active material based on an output from the SO 2 gas detection sensor; and a sequence controller configured to perform opening/closing operation of the plurality of solenoid valves in accordance with a predetermined sequence.