This study was conducted to establish a nondestructive inspection method based on ultrasonic echo measurement. The GFRP model material was immersed into hot water maintained at 371 K during 1 day to 28 days. The progress of damage was investigated from the surface and on the cross-section in detail. At the same time, ultrasonic echo was measured at all area of the material from the surface. As a parameter, the integrated value of echo level per unit time V^^- value was used. The results showed that the damage condition of the GFRP material such as density of delamination and its progress can be adequately evaluated by the distribution of V^^- value.