Abstract: Plural auxiliary carts 18 and 19 are connected to a measuring cart 11 in the width direction thereof which travels on a tank bottom plate 31 through universal joint mechanisms 26 and 27. Reflection type ultrasonic probes 22, 23 and coating film thickness gauges 24, 25 are provided with the respective auxiliary carts 18 and 19, and also a rotary encoder 32 which measures a traveling distance of the measuring cart 11 is attached to the measuring cart 11. Based on the outputs of the ultrasonic probes 22, 23, the outputs of the coating film thickness gauges 24, 25 and the outputs of the rotary encoder 32, the actual thickness of the tank bottom plate 31, that is obtained by subtracting the thickness of the coating film at a specific position of the tank bottom plate 31 measured by the rotary encoder, is measured and stored, and the thickness of every portion of the tank bottom plate 31 is displayed on a screen.