[Summary][Object] To accurately detect a state of a secondary battery when the secondary battery is mounted on a vehicle.[Organization] In a secondary battery state detecting device 1 detecting a state of a secondary battery 14 which is mounted on a vehicle, a first operation mode being an initial operation mode in a case newly connected to the secondary battery and a second operation mode being a normal operation mode after the first operation mode are included, and an initial state detecting unit (control part 10) which makes the operation mode transit to the first operation mode, and detects an initial state of the secondary battery with reference to a terminal voltage of the secondary battery in a case connected to the secondary battery; and a normal state detecting unit (control part 10) which makes the operation mode transit to the second operation mode, and detects the state of the secondary battery at a normal time based on the terminal voltage and a charge/discharge current of the secondary battery at a predetermined timing after the state of the secondary battery is detected by the initial state detecting unit are included.