A detector includes a measurement unit and a battery control unit. The measurement unit measures a measurement value indicating a battery state of a secondary battery. The battery control unit detects a sign of thermal runway in the secondary battery. The measurement unit determines whether a start-up signal is output based on the measurement value in a case where the battery control unit is in a sleep state, and outputs the start-up signal in a case where the start-up signal is output. The battery control unit starts by receiving the start-up signal from the measurement unit when the battery control unit is in the sleep state, executes arithmetic processing on the measurement value received from the measurement unit, and detects the sign of thermal runaway based on a calculated result of the arithmetic processing.