The invention relates to an apparatus based on the telecentric imaging system for forming an image of a linear zone (17) of an object (1). The apparatus comprises a non-telecentric camera (9) consisting of an objective (8) and an image plane (19) formed by a row of photosensitive cells (15) as well as telecentric imaging means (18) between the objective and the object. The telecentric means comprise a concave strip mirror (6), which is aligned with said row of cells and with the aperture of said objective located in its focal plane, as well as a strip-like plane mirror (5) between the parabolic mirror and the objective, whereby the radiation being reflected from the objective continues via the parabolic mirror and the planar mirror further to the objective and from there to the image plane. In the apparatus there is further a scattered light source (7) which lightens the object. The apparatus is adapted for measuring the dimensions of the width parts of the object.