PURPOSE: To surely detect the fine waviness of a sheet board by irradiating a surface to be measured with slit light having intense directivity in two directions and photographing and analyzing the projection image of the reflected light.

CONSTITUTION: Light having intense directivity in the two directions of light sources 1, 2 is changed into linear luminous flux by the combination of a lens system and a slit system, and a surface to be measured 8 is irradiated with the light at a fixed angle from an oblique upper section. The reflected light is projected onto light-receiving boards 3, 4 mounted at places opposed to the light sources 1, 2 respectively to the surface to be measured 8. The two projection images are picked up by CCD cameras 5, 6, and taken in by an image processor 7 as image data. Noises of each image data are removed, and binarized and processed and binary images are formed, and two images are analyzed by an arithmetic processing circuit. Linear distribution having no distortion is obtained in photographed images when the surface to be measured 8 is normal at that time. When there is waviness, the displacement of the waviness is magnified by the light-receiving boards, and a distortion image corresponding to waviness appears in at least one of the two images, thus positively detecting the waviness of the sheet board.