Volatile organic compounds (VOCs) are organic chemical substances that volatilize easily in ambient air at normal temperature and pressure. VOCs in human expired gas have been reported to be useful in the diagnosis of various diseases, but measurement of VOCs in human expired gas is technically difficult because the concentrations in expired gas are extremely low and almost the same as the concentrations in ambient air. Accurate VOC measurement usually requires a large system, and no VOC measuring systems suitable for clinical practice are available. We developed a compact, simple, double cold trap system that can measure the concentrations of VOCs originating in humans. Our system detects a limited number of VOCs with very high sensitivity at concentrations as low as 0.05 ppb. We evaluated the reproducibility of our system and measured VOCs in ambient air, purified air, and human expired gas from smokers, non-smokers, and patients. Errors of ±10% seem unavoidable in our system. Our verification experiment using human expired gas strongly suggests that the reproducibility and detection sensitivity of our system allow the detection of most VOCs in human expired gas.