Abstract: By arranging a pair of an ultrasonic wave transmitting sensor and an ultrasonic wave receiving sensor in opposition across a crack in a reinforced concrete construction. An ultrasonic wave is transmitted from the ultrasonic wave transmitting sensor at a first position. A reflected wave is then received by the ultrasonic wave receiving sensor at the first position. Also, the ultrasonic wave is transmitted from the ultrasonic wave transmitting sensor at a second position. A reflected wave is then received by the ultrasonic wave receiving sensor at the second position. The received waves at two positions are provided mutual phase shift for a predetermined period and summed in reversed phase for deriving a rise time of the wave reflected from the crack.