This invention provides a remote monitoring system for air conditioners installed respectively in a plurality of buildings for concentrically monitoring the operating state of the air conditioners from a distance. The system comprises data collectors 3 installed in the respective buildings, and a remote monitor 4 connected to the data collectors 3 of all the buildings via a communication network. Each of the data collectors 3 collects state data representing the operating state of the air conditioner installed in each building and transmits the collected state data to the remote monitor 4. The remote monitor 4 receives the state data collected by each data collector 3, diagnoses the operating state of the air conditioner installed in each building based on the received state data, and outputs the result of diagnosis. In this way, the air conditioners can be optimally adjusted within a short period of time.