Abstract: Disclosed is a monitoring probe for observing groundwater, which is put into an observation well for observing the groundwater to indicate a position of a fresh water-salt water interface of the groundwater. According to the present invention, both ends of a probe body are separated from an inner circumference of an observation well by a catch preventing portion having a convex curved portion or a dumbbell-shaped portion, so that the probe body can be smoothly moved up and down without being caught by a ground-water inlet port of the observation well, thus providing an exact position of a fresh water-salt water interface.