A monitoring device includes: a PUF-information acquiring unit acquiring PUF-information; a first PUF-information change value calculating unit calculating a degree of change from PUF-information acquired at a previous time to PUF-information acquired at a current time as a first PUF-information change value; a second PUF-information change value calculating unit calculating a degree of change from initial PUF-information to the PUF-information acquired at the current time as a second PUF-information change value; an unauthorized use determining unit comparing the first PUF-information change value with a first threshold and determining that there is unauthorized use of the predetermined monitoring target device when the first PUF-information change value is greater than the first threshold; and a deterioration determining unit comparing the second PUF-information change value with a second threshold and determining that the predetermined monitoring target device has deteriorated when the second PUF-information change value is greater than the second threshold.