Disclosed in an embodiment is an operating method of portable apparatus for noninvasively measuring blood glucose levels. The method includes: (a) measuring a first signal value according to ambient environmental light when an LED for measuring signals which emits light with wavelengths to be absorbed into or scattered by glucose is switched off； (b) measuring a second signal value according to light which is scattered by or transmitted through subject tissue and enters a photodetecting unit when the LED for measuring signals is switched on； (c) calculating a glucose concentration measurement of a subject by using the first signal value and the second signal value； and (d) adjusting the quantity of the light radiated from the LED for measuring signals by feeding the difference between the glucose concentration measurement and a pre-established first reference value back to the driving current for the LED for measuring signals.
