Chloride contents estimation using electromagnetic waves as a non-destructive method is effective to conceive degradation of concrete structure, no damage to actual structure and wide range condition would be easily evaluated. From the past study confirmed that the more chloride content is increased in the concrete, the less amplitude value of the electromagnetic waves get. Utilization of this characteristic enables to estimate chloride contents. Giving light on the attenuation characteristics of electromagnetic waves by material change of properties in concrete, and lead the fundamental equation which contributes to estimate chloride contents in concrete using electromagnetic waves. The estimation of chloride contents around reinforcing bars is reported