A protection circuit device for a battery having secondary batteries connected in series. A reference voltage circuit generates a reference voltage and a voltage detection circuit for detecting a voltage of one of the secondary batteries and comparing the detected voltage with the reference voltage generated by the reference voltage circuit to thereby detect an over-charge state and an over-discharge state of the one of the secondary batteries. Each of the reference voltage circuit and the voltage detection circuit has a power supply terminal connected to a positive electrode of the one of the secondary batteries and has a ground terminal connected to a negative electrode of the one of the secondary batteries. A withstand voltage of elements forming the reference voltage circuit and the voltage detection circuit is set to be lower than an overall voltage of the secondary batteries.