Abstract: To fill a cable connection head with an insulating oil and provide a movable lid member of the oil to ensure a long-term insulating property in the cable connection head and enable long-term continuous use of a measuring device. A cable connection head 18 having a head body 21 comprised of a bottom head body 19 and a top head body 20 connected to a measuring device body 6, having a well logging cable 1 passed through the top head body 30, and having core conductors of sheathed cables 2 connected to a connector 11 fit into the bottom head body 19, wherein the bottom head body 19 is filled with a low viscosity insulating oil 29 with a specific gravity heavier than the borehole fluid, outflow of the low viscosity insulating oil 29 is prevented, and a movable sealing member 30 preventing entry of borehole fluid is provided movably inside the bottom head body 19.

Abstract: [PROBLEM] To fill a cable connection head with an insulating oil and provide a movable lid member of the oil to ensure a long-term insulating property in the cable connection head and enable long-term continuous use of a measuring device. [MEANS FOR SOLUTION] A cable connection head 18 having a head body 21 comprised of a bottom head body 19 and a top head body 20 connected to a measuring device body 6, having a well logging cable 1 passed through the top head body 30, and having core conductors of sheathed cables 2 connected to a connector 11 fit into the bottom head body 19, wherein the bottom head body 19 is filled with a low viscosity insulating oil 29 with a specific gravity heavier than the borehole fluid, outflow of the low viscosity insulating oil 29 is prevented, and a movable sealing member 30 preventing entry of borehole fluid is provided movably inside the bottom head body 19.