An activity monitoring system and method are disclosed to reduce the size of an activity sensor device to reduce its power consumption and smoothly transmit activity and occupancy information from the activity sensor device and an occupancy sensor device to an activity monitoring server to monitor an individual's activity and occupancy information. The activity monitoring system includes: an activity sensor device configured to detect a physical activity of a user, classify the detected physical activity, and transmit activity classification information as a result along with user identification information; one or more occupancy sensor devices configured to receive the activity classification information and the individual's identification information from the activity sensor device, and transmit the received information along with occupancy information; and an activity monitoring server configured to manage the activity classification information, the occupancy information, and the individual's identification information received from the occupancy sensor devices.