A controller includes a calculator configured to receive process data necessary for controlling a plant and vital data indicating a state of a plant worker, the process data being received from a field device, the vital data being received from the plant worker, the calculator calculating first index data and second index data, the first index data indicating a gap between a current one of the process data and a previous one of the process data which is received when the plant is in a normal state, the second index data indicating a gap between a current one of the vital data and a previous one of the vital data which is received when the plant is in the normal state, and a determiner configured to determine health of the plant by using the first index data and the second index data which are calculated by the calculator.