A battery state detection device for a battery system including a battery module with series-connected battery cells is disclosed. The device comprises at least one of a circuit or a processor with memory storing executable program code. The device is configured to acquire state detection values of each battery cell, use either the state detection value or a calculation value derived from it as a state parameter, and calculate a parameter change amount over a predetermined period. Each battery cell is sequentially set as a target for abnormality detection, and an abnormality is detected based on whether a predetermined deviation occurs in the parameter change amount of the target cell relative to another battery cell.