To provide a monitoring system suitably monitoring health conditions of all man users wearing sensor devices. Each of sensor devices includes a first measurement section attached to users and measuring first biological information a the users, a storage section storing information for identifying the users, an analysis section analyzing a measurement result of the first measurement section to determine that the measurement result matches with a first condition and a communication section transmitting a determination result indicating the match with the first condition received from the analysis section and information for identifying the users to communication devices located every same interval.