A secondary battery protection circuit for a secondary battery includes: a reference voltage circuit configured to generate a reference voltage by using a depletion-type transistor and a transistor unit of enhancement type connected in series with the depletion-type transistor; a voltage divider configured to output a detection voltage obtained by dividing a power source voltage of the secondary battery; a detection circuit configured to detect an abnormal state of the secondary battery based on the reference voltage and the detection voltage; a first adjustment circuit configured to adjust a size ratio of the depletion-type transistor to the transistor unit based on threshold voltages of the depletion-type transistor and the transistor unit; and a second adjustment circuit configured to adjust the detection voltage based on the reference voltage after adjusting the size ratio.