PURPOSE: To enhance detection capacity by increasing the contrast of an image, by picking up the image of a tire tube plural times in a stretched state and adding a plurality of obtained image signals to form an image signal.

CONSTITUTION: A tire tube 1 to be inspected is provided so as to be extended to the longitudinal direction to be mounted to first and second clampers 2a, 2b and these clampers 2a, 2b are moved to stretch the tire tube 1 to the longitudinal direction thereof. The tire tube 1 is irradiated with X-rays from an X-ray detector in this stretched state and transmitted X-ray image is obtained on an X-ray camera 5. Further, image signals obtained plural times by the X-ray camera 5 are added by an image processor 7 to be displayed on a television monitor 8.