PURPOSE: To shorten surface-to-surface distance and to facilitate attachment to and detachment from a duct by preventing the leakage of magnetic flux generating at an energizing part through a main body casing, and applying flangeless structure to the casing.

CONSTITUTION: A main body casing 1 is made of a magnetic body and both end parts of a measuring pipe 4 are supported in a circumscribed state by mirror plates 1a and 1b at both ends of the casing. The internal surface of the measuring pipe 4 is lined as shown by 3. An energizing part 2 is equipped with a yoke 2b and an energizing coil 2a provided in a cover 1c constituting part of the shell part of the casing 1. To the measuring pipe 4, a couple of electrodes 5 are fitted. A flange clamping bolt runs through a bolt insertion hole 10, bored in the internal surface of the shell part of the casing 1, and the yoke 2b to be inserted into a pipe 12 made of a nonmagnetic material provided between both the mirror plates 1a and 1b.