PURPOSE: To provide the image pickup device of high resolution without increasing the area of an image sensor by providing an optical path change means which forms the image of a part along the arbitrary horizontal line of an object on the linear image sensor.

CONSTITUTION: The image of the part along the horizontal line on the prescribed vertical position 1A(1) of the object 1 is reflected on the plane of one plane mirror 5A of a polygon mirror, and it is collected on the light-receiving surface 4A of the linear image sensor 4 by a convex lens 6. The image is converted into an electric signal and is outputted as image data. Image data for one line is accumulated in an image processing part. Then, the polygon mirror 5 rotates by a prescribed angle, the surface of the plane mirror 5A is shifted and the image of the next subject 1A(2) is condensed on the light- receiving surface 4A of the image sensor 4. An image processing part accumulates one-dimensional image data transmitted from the image sensor 4 for one frame and converts it into two-dimensional image data for one frame.