PURPOSE: To exactly measure a coating part whose coating thickness is thin by a few sensors, by measuring a coating thickness distribution extending over the whole area between coating pitches by the sensors consisting of 2 pieces and 1 row, and for detecting the coating thickness, and moving all the sensors for detecting the coating thickness, to the minimum value side of said each measured coating thickness.

CONSTITUTION: A screw shaft 4 is rotated by a reversible motor 9, and coating thickness detecting sensors 7, 8 are scanned together with a sensor supporting arm along the lengthwise direction of a synthetic resin coating steel pipe 1', by which a coating thickness distribution extending over the whole area between coating pitches is measured. Each measured value of the coating thickness distribution measured by the 2 coating thickness detecting sensors 7, 8, respectively is compared and operated by a comparing operator 10, and the coating thickness detecting sensors 7, 8 are always moved to a side having a small measured value of the coating thickness distribution. A smaller value of each coating thickness measured value measured by the coating thickness detecting sensors 7, 8 is sent to a recorder 11 through the comparing operator 10, and is recorded.