A failure detection device for a battery pack having a plurality of battery cells (CLs) connected in series, comprising: a plurality of detection units associated with said plurality of battery cells, respectively, and each configured to compare a voltage output from a battery cell associated therewith with a predetermined criterion voltage; a current detector configured to detect a current of said plurality of battery cells, said plurality of detection units configured to sequentially transmit a signal that reflects a result of comparing said voltages so as to output a fault detection signal indicative of whether said battery back when electrically discharged has any of said battery cells outputting a voltage decreased to be lower than said criterion voltage; a fault monitoring device configured to monitor whether an internal resistance fault has occurred based on said fault detection signal and a value of said current detected by said current detector, said internal resistance fault indicating that any of said battery cells has an internal resistance increasing to be higher than an upper limit value, characterized in that said plurality of detection units being configured to operate sequentially in response to a start trigger, and the detected current, which is compared with the criterion current in the fault monitoring device, is a value of the current detected at the time indicated by the fault detection signal that an output voltage of any of the battery cells is lower than the criterion voltage at the time of discharge of the battery pack, and said fault monitoring device being configured to detect said internal resistance fault when said fault detection signal indicates that said battery pack, when electrically discharged, has any of said battery cells outputting a voltage decreased to be lower than said criterion voltage and if said value of said current detected is smaller in magnitude than a criterion current corresponding to a value of a current obtained by dividing a difference in voltage between an open circuit voltage of said battery cell and said criterion voltage by said upper limit value for said internal resistance. a plurality of detection units associated with said plurality of battery cells, respectively, and each configured to compare a voltage output from a battery cell associated therewith with a predetermined criterion voltage; a current detector configured to detect a current of said plurality of battery cells, said plurality of detection units configured to sequentially transmit a signal that reflects a result of comparing said voltages so as to output a fault detection signal indicative of whether said battery back when electrically discharged has any of said battery cells outputting a voltage decreased to be lower than said criterion voltage; a fault monitoring device configured to monitor whether an internal resistance fault has occurred based on said fault detection signal and a value of said current detected by said current detector, said internal resistance fault indicating that any of said battery cells has an internal resistance increasing to be higher than an upper limit value, characterized in that said plurality of detection units being configured to operate sequentially in response to a start trigger, and the detected current, which is compared with the criterion current in the fault monitoring device, is a value of the current detected at the time indicated by the fault detection signal that an output voltage of any of the battery cells is lower than the criterion voltage at the time of discharge of the battery pack, and said fault monitoring device being configured to detect said internal resistance fault when said fault detection signal indicates that said battery pack, when electrically discharged, has any of said battery cells outputting a voltage decreased to be lower than said criterion voltage and if said value of said current detected is smaller in magnitude than a criterion current corresponding to a value of a current obtained by dividing a difference in voltage between an open circuit voltage of said battery cell and said criterion voltage by said upper limit value for said internal resistance.