A protector for an electricity supply circuit, which is installed on a vehicle to monitor a temperature of the electricity supply circuit connected to a load and which disconnects the electricity supply circuit when the temperature of the electricity supply circuit exceeds a preset threshold temperature, and thereby to protect the electricity supply circuit, the protector comprising: a power switch capable to switch between connection and disconnection of the electricity supply circuit; a controller configured to output a switching command signal to the power switch in accordance with an input signal; and a current detector for detecting current flowing to the electricity supply circuit, wherein the controller comprises: a temperature estimation unit configured to estimate rising temperature of the electricity supply circuit based on a current detected by the current detector and conducting time when the electricity supply circuit is turned on, estimate lowering temperature of the electricity supply circuit based on elapsed time when the electricity supply circuit is turned off by the controller, and estimate temperature of the electricity supply circuit based on the rising temperature and the lowering temperature; a timer for counting elapsed time that passes after the load has been turned off when the load is turned off by the power switch; a lower limit threshold determination unit configured to determine whether or not the estimated temperature of a power line estimated by the temperature estimation unit has dropped below a predetermined temperature determined by ambient temperature of the electricity supply circuit; and a mode switching unit configured to switch the controller to a low power consumption mode where the power consumption is smaller than that in a normal operation mode when a predetermined time is counted by the timer and the lower limit threshold determination unit determines that the estimated temperature of the power line estimated by the temperature estimation unit drops below the predetermined temperature. a power switch capable to switch between connection and disconnection of the electricity supply circuit; a controller configured to output a switching command signal to the power switch in accordance with an input signal; and a current detector for detecting current flowing to the electricity supply circuit, wherein the controller comprises: a temperature estimation unit configured to estimate rising temperature of the electricity supply circuit based on a current detected by the current detector and conducting time when the electricity supply circuit is turned on, estimate lowering temperature of the electricity supply circuit based on elapsed time when the electricity supply circuit is turned off by the controller, and estimate temperature of the electricity supply circuit based on the rising temperature and the lowering temperature; a timer for counting elapsed time that passes after the load has been turned off when the load is turned off by the power switch; a lower limit threshold determination unit configured to determine whether or not the estimated temperature of a power line estimated by the temperature estimation unit has dropped below a predetermined temperature determined by ambient temperature of the electricity supply circuit; and a mode switching unit configured to switch the controller to a low power consumption mode where the power consumption is smaller than that in a normal operation mode when a predetermined time is counted by the timer and the lower limit threshold determination unit determines that the estimated temperature of the power line estimated by the temperature estimation unit drops below the predetermined temperature. a temperature estimation unit configured to estimate rising temperature of the electricity supply circuit based on a current detected by the current detector and conducting time when the electricity supply circuit is turned on, estimate lowering temperature of the electricity supply circuit based on elapsed time when the electricity supply circuit is turned off by the controller, and estimate temperature of the electricity supply circuit based on the rising temperature and the lowering temperature; a timer for counting elapsed time that passes after the load has been turned off when the load is turned off by the power switch; a lower limit threshold determination unit configured to determine whether or not the estimated temperature of a power line estimated by the temperature estimation unit has dropped below a predetermined temperature determined by ambient temperature of the electricity supply circuit; and a mode switching unit configured to switch the controller to a low power consumption mode where the power consumption is smaller than that in a normal operation mode when a predetermined time is counted by the timer and the lower limit threshold determination unit determines that the estimated temperature of the power line estimated by the temperature estimation unit drops below the predetermined temperature.