A protector for an electricity supply circuit includes: a power switch (11) capable to switch between connection and disconnection of the electricity supply circuit; a controller (12) configured to output a switching command signal to the power switch (11) in accordance with an input signal; and a current detector (11) for detecting current flowing to the electricity supply circuit. The controller (12) includes: a lower limit threshold determination unit (27) configured to determine whether or not an estimated temperature of a power line has dropped below a lower limit threshold; a timer (22) for counting the time that passes after a load (RL) has been turned off, when the load (RL) is turned off by the power switch (11); and a mode switching unit (23) configured to switch the controller (12) to a sleep mode when a predetermined time (Q1) is counted by the timer (22) and the lower limit threshold determination unit (27) determines that the estimated temperature of the power line has dropped below the lower limit threshold.