To provide an authentication system and a semiconductor device utilizing the system.The semiconductor device includes a transmission/reception circuit, a control circuit, an analog-to-digital converter circuit, a memory device, and a fingerprint sensor. At least one of the control circuit, the analog-to-digital converter circuit, and the memory device includes a transistor including an oxide semiconductor in a channel formation region. The control circuit has a function of receiving an instruction signal from the outside of the semiconductor device through the transmission/reception circuit. The memory device has fingerprint data for comparison and confidential information. The control circuit has a function of comparing fingerprint data to be compared which is obtained by the fingerprint sensor and the fingerprint data for comparison. The control circuit has a function of transmitting the confidential information to the outside of the semiconductor device through the transmission/reception circuit when the fingerprint data to be compared and the fingerprint data for comparison match each other.