PURPOSE: To extract a profile line of an object wheel exists originally regardless of quality of an input picture by tracing profile candidate points in accordance with three types of different picture data such as several picture elements including profile condidate points, those at the outside of said points and those at the inside.

CONSTITUTION: Features of an object to be detected are extracted from current picture data in an RAM array 5. A central position P0 (X0, Y0) of a circle is obtained as a characteristic point. Then profile candidate points are estimated from said point P0 and a radius of a circle. For convenience, each candidate point is expressed by Pi (i=1Wn). Continuously several picture elements Ci including profile candidate points Pi, several picture elements Oi at the outside of said points Pi and several picture elements Ii at the inside of said points Pi are extracted. Then means values of brightness of three types of picture elements such as the Ci, Oi and Ii are obtained. Differences from obtained means are obtained, and if three differences are beyond the permissible range, a distance between adjoining profile candidate points is decided to be discontinuous. If any of differences is within said range, a distance between adjoining profile candidate points is decided to be continuous.