A detection device (2) is retained in the rumen (3) of a cow (1) by being orally administered to the cow (1), and detects the state of the inside of the rumen (3). The detection device (2) wirelessly transmits measured values of the rumen pH as detection results. A monitoring unit (receiver (4) and monitoring server (7)) acquires information transmitted from the detection device (2), and monitors the state of the inside of the rumen (3). The detection device (2) is configured to be recoverable orally from the rumen (3) of the cow (1). The operating conditions of the detection device (2) are recorded in the detection device (2) ahead of time, and can be updated by means of a wireless signal transmitted from a setting unit (12) to the detection device (2).
