PURPOSE: To extend a durability life of a seal member by supplying a working pressure to a working region and supplying a lower counter pressure to an intermediate region and an end part region in both sides of a working region.

CONSTITUTION: Working regions (a1), (a2) are set by seal members 7-10 arranged on sleeves 3, 4 in a pairing state. A center region (b2) is positioned between these working regions. A seal member 11 is arranged on a sonde head 2, an end region (c) is defined between seal members 10, 11. The working regions (a1), (a2) are communicated to a pressure medium supply hole 14 in a sonde main body 1 by radial holes 12, 13, the supply hole 14 piercing the sonde main body 1 is closed in the sonde head 2 by a plug 15. Intermediate regions (b1), (b2) are communicated to a longitudinal passage formed as a groove in a round side tube 5 through radial holes 16, 17, the end region (c) is communicated to the longitudinal passage through an another radial hole 19, the longitudinal passage itself is closed in the sonde head 2. This second hole system is used for generating a counter pressure in the whole regions of the intermediate region (b) and the end region (c).