Concrete float is known to be caused by corrosion and expansion of reinforcing bars. However, many cases of re-deterioration after repair have been confirmed. It is speculated it is mainly due to impromptu repairs that ignore the float generation mechanism. The purpose of this study is to estimate the internal defect shape of concrete and to clarify the mechanism of float generation by using the infrared method and three-dimensional unsteady heat transfer analysis, which are one of the non-destructive inspection techniques. In this paper, a method for estimating the defined position of the sound part, the shape, range and depth of the float was clarified for concrete with defects.