Functional residual capacity (FRC) is an important diagnostic parameter measured using analyzer. Since analyzer is expensive as well as cumbersome for use of noisy vacuum pump, the FRC measurement becomes possible only in large well-equipped hospitals. The present study introduced a new wash-out technique to measure FRC by analysis, which is relatively cheaper and much simpler to apply. Slower response was compensated for high frequency to be coincided with response, thereby enabled indirect, but accurate concentration measurement. FRC was estimated by continuous integration of expired volume obtained with air flow signal. Experiment with 3 L syringe, a standard calibration device recommended by the American Thoracic Society, demonstrated less than 1% error at 0, 1, and 2 L. Correlation coefficient was almost ideal, guaranteeing linear estimation of FRC. The present technique is inexpensive and simple to apply, thus should he of great convenience.