A measuring apparatus for an ear thermometer comprising: a common voltage line a built-in battery serving as a power source a connector to receive a connector of the probe, having a common voltage terminal connected to the common voltage line a flash-type microcontroller to control a temperature sensor of the probe, receive a resistance value output signal corresponding to a measured temperature from the temperature sensor, convert the signal into a digital temperature value, and output the digital temperature value, the microcontroller having a test port, a program write port, and a common voltage port connected to the common voltage line, establishing a flash mode when a HIGH voltage higher than a first predetermined voltage is applied to the test port, to enable a program to be written through the write port, and establishing a run mode when a LOW voltage lower than the first predetermined voltage is applied to the test port a voltage regulator having an input side connected to the common voltage line, to provide a constant reference voltage and a mode switching circuit connected to the common voltage line, to apply the HIGH voltage to the test port of the microcontroller when a common voltage is higher than a second predetermined voltage, apply the LOW voltage to the test port of the microcontroller when the common voltage is lower than the second predetermined voltage, and bypass a leakage current passing from the common voltage line to the mode switching circuit toward an output of the voltage regulator so as to combine them together, the connector having the common voltage terminal, a battery power source terminal connected to the built-in battery, a program write terminal connected to the write port of the microcontroller, and a sensor connection terminal to receive the resistance value output signal corresponding to a measured temperature from the temperature sensor of the probe.
