Abstract: A detection device is retained in the rumen of a cow by being orally administered to the cow, and detects the state of the inside of the rumen. The detection device wirelessly transmits measured values of the rumen pH as detection results. A monitoring unit (receiver and monitoring server) acquires information transmitted from the detection device, and monitors the state of the inside of the rumen. The detection device is configured to be recoverable orally from the rumen of the cow. The operating conditions of the detection device are recorded in the detection device ahead of time, and can be updated by means of a wireless signal transmitted from a setting unit to the detection device.