To provide a diagnostic system and diagnostic technique of concrete structure capable of attaching an oscillator easily to diagnose a concrete structure.

The diagnostic system includes a magnetic substance sheet member adhered to a concrete structure, an oscillator fixed adsorptively to magnetic substance sheet member magnetically during the period of operating condition to give local oscillation to the concrete structure to be diagnosed, a driving means for driving the oscillator, a measuring means measuring response to oscillation of the concrete structure to be diagnosed, and an analyzing means for determining soundness of the concrete structure to be diagnosed, based on how the oscillation mode obtained from measurement of a measuring means is varied temporally from oscillation mode at the time of sound condition.