This paper describes a nondestructive inspection method for estimating compressive strength of concrete using velocity of impact-elastic wave, and explains influence of variations of concrete materials and mix proportions on relation between the velocity and the strength for improving the accuracy of estimation. First, we survey the recent studies about methods to measure velocity of impact-elastic wave and relations between the velocity and compressive strength. Next, experiments to clear influential factors on the velocity, such as W/C, slump, bulk volume of coarse aggregate and cement type, were carried out using cylinder specimens. The results to describe influence of the four factors on relation between elastic wave velocity and compressive strength were presented. Finally, an appropriate estimating formula was given by fitting to the experimental relation.