A monitoring system monitors a monitoring object apparatus sufficiently for practical use while suppressing battery power consumption. The monitoring system is provided a sensor terminal that is driven by a battery and acquires a measured value(s) of a measuring apparatus mounted in the monitoring object apparatus by executing image analysis processing on a captured image(s) obtained by photographing the measuring apparatus. A manager terminal monitors the measured value(s) of the measuring apparatus which is reported from the sensor terminal. When the measured value is out of a normal range, the manager terminal requests transmission of image data of an image area, which is necessary to check whether the measured value of the measuring apparatus is actually out of the normal range or not. The sensor terminal transmits the requested image data with the requested resolution to the manger terminal in response to the request from the manager terminal.