We developed a mid-IR laser absorption spectrometer for ambient carbon monoxide (CO) measurement that uses a 4.69 mu m interband cascade laser with wavelength modulation spectroscopy. The CO concentration was measured with a reproducibility of 0.4% (1 sigma) over a 7-h period. Measurements of CO in outside air with a temporal resolution of 1 s were conducted successfully on the Kashiwa campus of The University of Tokyo over 4 days without trouble.