The invention is aimed that the soundness (degree of deterioration) of a tunnel can be inspected according to each span. The image synthesis processing is performed to obtain the image showing both sides of the tunnel lining surface according to the each span of the tunnel lining surface, by comparing the first image photographied by the photography means, showing one side face in both side faces of the tunnel lining surface, while the photography means is fixed to the first photography position, and the second image photographied by the photography means, showing the other side face in both side faces of the tunnel lining surface, while the photography means is fixed to the second photography position, and indicating the portions forming the identical span of the tunnel lining surface.