PURPOSE: To measure easily the field intensity under overhead transmission and distribution lines, by fixing an expansible insulating operation bar, where a field intensity detector is fixed to the tip, to a movable truck and placing a gas supply means on the truck.

CONSTITUTION: A truck 1 moved to a field intensity measurement position is fixed by a stopper 3, and a gaseous N2 bomb 19 is connected to an air hose 14. Gaseous N2 is fed into an expansible insulating operation bar 5 through the air hose 14 and a gas seal hole 17 to expand the operation bar 5. When the height of a field intensity detecting part 6 attached to the tip part of the operation bar 5 reaches a prescribed height, a reducing valve 18 is closed to stop the detecting part 6 at this height, and then, the field intensity in a prescribed position under overhead transmission and distribution lines 21 is displayed on a display device 7. After the measurement, a valve 15 is opened, and a rope 8 is wound up be a roll take-up reel 11, and then, gaseous N2 sealed in the operation bar 5 is discharged from a gas discharge pipe 16, and the operation bar 5 is contracted.