A protector for an electricity supply circuit, wherein the protector is installed on a vehicle to monitor a temperature of the electricity supply circuit connected to a load and, wherein the protector is able to disconnect the electricity supply circuit when an estimated temperature of the electricity supply circuit exceeds a preset threshold so 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 characterized by: a temperature estimation unit configured to calculate rising temperature of the electricity supply circuit based on a current detected by the current detector and conducting time since when the electricity supply circuit is turned on, calculate lowering temperature of the electricity supply circuit based on elapsed time since when the electricity supply circuit is turned off by the controller, and calculate 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; 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. 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 characterized by: a temperature estimation unit configured to calculate rising temperature of the electricity supply circuit based on a current detected by the current detector and conducting time since when the electricity supply circuit is turned on, calculate lowering temperature of the electricity supply circuit based on elapsed time since when the electricity supply circuit is turned off by the controller, and calculate 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; 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. a temperature estimation unit configured to calculate rising temperature of the electricity supply circuit based on a current detected by the current detector and conducting time since when the electricity supply circuit is turned on, calculate lowering temperature of the electricity supply circuit based on elapsed time since when the electricity supply circuit is turned off by the controller, and calculate 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; 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.