A detection system including a detection device for detecting an internal state of a rumen of a ruminant and a recovery instrument, said detection device comprising: a housing formed of a material having resistance to liquid component of contents in said rumen and having physical strength to withstand movement and inner pressure of said rumen, configured to enable oral administration to said ruminant and engageable with a recovery instrument orally inserted to the inside of said rumen; wherein said housing has an opening for introducing said liquid component to a first chamber inside said housing and a coupling portion formed of at least one of ferromagnetic body and paramagnetic body; a separating portion for sectioning the inside of said housing to said first chamber and a second chamber not permitting entrance of said liquid component; a measuring unit (21, 21A) placed in said first chamber of said housing for measuring a parameter related to said liquid component; a storage unit placed in said second chamber of said housing for storing information related to operational conditions of said detection device; a control unit placed in said second chamber of said housing for generating data related to said parameter from result of measurement by said measuring unit (21, 21A), based on said information stored in said storage unit; a communication unit (25, 26, 26A) placed in said second chamber of said housing for transmitting through wireless communication said data related to said parameter generated by a process by said control unit; a battery (32, 32A) placed in said second chamber, for supplying electric power at least to said control unit and said communication unit (25, 26, 26A); wherein said recovery instrument is configured to be inserted to the inside of the rumen and includes a magnet for taking out the detection device from the body of the ruminant; wherein said coupling portion of said detection device is configured to be coupled to the magnet of said recovery instrument. a housing formed of a material having resistance to liquid component of contents in said rumen and having physical strength to withstand movement and inner pressure of said rumen, configured to enable oral administration to said ruminant and engageable with a recovery instrument orally inserted to the inside of said rumen; wherein said housing has an opening for introducing said liquid component to a first chamber inside said housing and a coupling portion formed of at least one of ferromagnetic body and paramagnetic body; a separating portion for sectioning the inside of said housing to said first chamber and a second chamber not permitting entrance of said liquid component; a measuring unit (21, 21A) placed in said first chamber of said housing for measuring a parameter related to said liquid component; a storage unit placed in said second chamber of said housing for storing information related to operational conditions of said detection device; a control unit placed in said second chamber of said housing for generating data related to said parameter from result of measurement by said measuring unit (21, 21A), based on said information stored in said storage unit; a communication unit (25, 26, 26A) placed in said second chamber of said housing for transmitting through wireless communication said data related to said parameter generated by a process by said control unit; a battery (32, 32A) placed in said second chamber, for supplying electric power at least to said control unit and said communication unit (25, 26, 26A); wherein said recovery instrument is configured to be inserted to the inside of the rumen and includes a magnet for taking out the detection device from the body of the ruminant; wherein said coupling portion of said detection device is configured to be coupled to the magnet of said recovery instrument.