An abnormal voltage detector apparatus is provided for an assembled battery including a plurality of battery blocks connected in series to each other. In the abnormal voltage detector apparatus, a detecting part detects whether or not each of the battery blocks is in a voltage abnormality state by comparing either one of a voltage of each battery block and each battery measuring voltage, that is a voltage lowered from the voltage of each battery block, with a predetermined reference voltage, generates each of abnormality detecting signals containing information about a detected result, calculates a time ratio of a time interval, for which the assembled battery is in a voltage abnormality state, to a predetermined time interval, based on the abnormality detecting signals, and detects a voltage abnormality of the assembled battery based on a calculated time ratio.