We propose one geometric algorithm to solve both the problem of circularity test and the problem of circularity measure. The former problem consists in deciding whether a given digital curve is a digitization of a circle or not, whereas the latter problem consists in measuring how far a given digital curve is from a digitization of a circle. Our new circularity measure fulfills basic invariance properties and equals 0 for all digital circles and is positive for all shapes that are not digital circles. Experiments performed on synthetic and real images are very good.