A protection of battery system having battery cells Cell1Cell3, FET switches SW1, SW2 that are connected to the high-side path and control on/off state of the path, as well as fuse F1 for cutting said path, primary protection circuits 3134 that detect abnormalities in charging/discharging of the battery cells and turn the FET switches off, and secondary protecting controller 38 that detect abnormalities in charging/discharging of the battery cells and controls the operation of fuse F1. Secondary protecting controller 38 controls fuse F1 if there is no tendency to a decrease in abnormalities after a prescribed period of time after control of the FET switch.