An odor-sensing system based on the insect olfactory receptor (OR) and fluorescent instrumentation has been developed. Cells expressing ORs were responsible for odor detection by monitoring the change in fluorescent light intensity emitted by the cells. Since fluorescent light from the cells is very weak, the measurement quality deteriorates if ambient light is present. A lockin measurement technique was incorporated to the system to overcome the problem of the ambient light together with the fluorescent image. The lock-in technique enables the detection of light synchronous with the modulated excitation light under the ambient light. This technique is useful for improving the signal-to-noise ratio. In this paper, sensitivity improvement by applying the lockin technique to fluorescent instrumentation for a cell-based odor sensor is discussed quantitatively. Experimental results show that the employment of the lock-in technique enhances the sensitivity by three orders of magnitude under the ambient light condition.