To install an electrode in an accurately positioned state in a parasympathetic nerve system which runs in a bile duct minimally invasively.

A nerve stimulator 1 includes: an almost cylindrical stent 2 which is insertable in a body cavity while contracting in a radial direction and can press the body cavity inner wall outward in the radial direction while expanding in the radial direction; two electrodes 3a, 3b installed on an outer peripheral surface of the stent 2 at an interval in a longitudinal direction; a stimulation circuit 4 which is provided in the stent 2 and outputs a stimulus signal to between the electrodes 3a and 3b; and a body cavity internal radius detection part 5 which is provided in the stent 2 and detects a change in inner diameter size of the body cavity corresponding to an output of the stimulus signal from the stimulation circuit 4.