To provide a method for finding an effective pore volume and a water-cement ratio so as to evaluate easily the quality of a concrete in an existing structure without being affected by an aggregate in the concrete.

An operation expression for finding a pore volume and an operation expression for finding a water-cement ratio based on a corrected ultrasonic propagation speed of a concrete are preliminarily found, the propagation speed of a measuring objective concrete is measured, an aggregate amount in the concrete and an ultrasonic propagation speed of an aggregate are found, a calculated value of a corrected ultrasonic propagation speed is found by an operation using the aggregate amount and the ultrasonic propagation speeds of the aggregate and the concrete, and the pore volume and the water-cement ratio are found using the respective operation expressions preliminarily found based on the calculated value hereinbefore.