The invention provides a monitoring device for a secondary battery, a protection system, a battery pack, and a vehicle for the secondary battery to obtain safety performance suitable for a use state. The monitoring device (BM) for an assembled battery (30) includes a detection part that detects a use state of the assembled battery (30), and a switching part. The switching part switches a protection condition of the assembled battery (30) in accordance with the use state of the assembled battery (30). Since the protection condition is switched, based on the use state, safety performance suitable for the use state of the battery can be assured.