A method for adjusting a trip sensitivity of a thermal overload protection apparatus, the thermal overload protection apparatus comprising bimetals for providing a driving force for trip operation by being bent when an overcurrent is conducted on a circuit, a shifter mechanism for transferring the driving force from the bimetals by contacting the same, a trip mechanism rotatable to a trip position at which the circuit is broken at a time of release, a trip latch mechanism movable to a position for releasing the trip mechanism from a position for restricting the trip mechanism by the driving force from the shifter mechanism, and an adjusting knob for adjusting a gap between the shifter mechanism and the trip latch mechanism, the method comprising: setting a position of an adjusting reference point for the adjusting knob; measuring a position of the bimetals when a normal current is conducted on the circuit; measuring a moving distance of the trip latch mechanism at the time of trip occurrence by arbitrarily moving the same in a direction of trip operation; determining an assembling position for the shifter mechanism based on the measured position and measured moving distance when the trip latch mechanism performs the trip operation, information on a predetermined trip distance between the shifter mechanism and the trip latch mechanism, and information on a size of the shifter mechanism; assembling the shifter mechanism at the determined assembling position; conducting a predetermined over current to the thermal overload protection apparatus; measuring a conducting time for the overcurrent until a trip occurrence; calculating a rotation angle by converting difference between measured conducting time and predetermined trip time into rotation angle; and marking a graduation of the trip operation current from the position of the adjusting reference point initially set in the setting step to a position adjusted by the rotation angle calculated in the calculating step. setting a position of an adjusting reference point for the adjusting knob; measuring a position of the bimetals when a normal current is conducted on the circuit; measuring a moving distance of the trip latch mechanism at the time of trip occurrence by arbitrarily moving the same in a direction of trip operation; determining an assembling position for the shifter mechanism based on the measured position and measured moving distance when the trip latch mechanism performs the trip operation, information on a predetermined trip distance between the shifter mechanism and the trip latch mechanism, and information on a size of the shifter mechanism; assembling the shifter mechanism at the determined assembling position; conducting a predetermined over current to the thermal overload protection apparatus; measuring a conducting time for the overcurrent until a trip occurrence; calculating a rotation angle by converting difference between measured conducting time and predetermined trip time into rotation angle; and marking a graduation of the trip operation current from the position of the adjusting reference point initially set in the setting step to a position adjusted by the rotation angle calculated in the calculating step.