PURPOSE: To monitor the temperature of an electric line to be monitored which has a small coefficient of thermal conductivity and is constituted so that a temperature difference can be generated between its internal temperature and surface temperature by measuring the surface temperature by monitoring the temperature of the electric line by adding the temperature difference to the measured surface temperature value of the electric line.

CONSTITUTION: After initialization (i), the time is measured (ii) and various kinds of data, such as conduction current In, etc., are fetched through sampling after Δt has elapsed (iii). Then, the quantities of generated heat Qi and Qs and the time constant Tsn of surface temperature variation of an electric line to be monitored are found (iv) and estimated values Qin and Qsn of the internal temperature and surface temperature of the electric line are found (v). Next, the quantities of generated heat Qi and Qs are compared with each other (vi) and, when Qi>Qs, the temperature Qn of the electric line is found by adding the temperature difference Qin-Qsn between the estimated temperatures Qin and Qsn to the measured surface temperature value of the electric line (vii). Then, maintenance information, etc., are outputted based on the monitored results (x).