A portable NDIR breath acetone measurement apparatus includes a portable housing formed with an optical cavity, which is housed in a receiving chamber for containing a gas sample taken from an exhaled breath of a human subject. A control unit controls the temperature in the receiving chamber to a desired temperature, and controls an IR emitter disposed in the receiving chamber to stably emit IR spectrum radiation propagating along a zigzag path toward an IR detector, which is operated at the desired temperature to generate, in response to detection of the IR spectrum radiation, an IR signal that is processed and analyzed by a signal processing unit to obtain a measurement result indicative of the concentration of acetone gas contained in the gas sample.