PURPOSE: To reduce the manpower of maintenance and monitor work by auto matically and accurately extracting only transnmission lines included in an outdoor picture.

CONSTITUTION: Picture information obtained by an ITV camera is divided into plural picture elements which are vertically and horizontally arranged to constitute a picture plane (step 105), and the brightness of a minute part of picture information corresponding to each picture element is converted to digital data (step 107). After this digital data is binarized by a prescribed threshold (step 109), a part where the change of brightness has a fixed periodicity with respect to each picture element row or column is extracted as a transmission line part from binarized data (steps 111 and 113). This extraction is repeated, and parts where the similar periodicity is recognized in adjacent picture element rows or picture element columns are collected and are recognized as a transmission line (step 119).