Strength of concrete in actual structures is often judged lower than that used for structural design. It is considered to occur due to the illegal addition of water during casting procedure, insufficient compaction during construction, deterioration due to aging and severe atmosphere, etc. Such low strength concrete does not make harmful effect to a performance of structure as long as it is subjected to only service loads. Performance of structures in severe condition e.g. in large earthquakes, however, is easily predicted to be greatly affected by the strength of concrete. In the present study, a series of nondestructive tests has been carried out for finding the low strength and therefore low quality concrete in site. Testing items include scratched width by a steel nail, rebound number by a test hammer, ultrasonic velocity, etc. As a result, low quality concrete in structure may be found out rather easily by using such nondestructive testing methods. It is also pointed out that the scratching test is quite effective and easily handled considering the work in site.