In recent years, levees have been collapsing and residents have been being robbed of their lives and properties almost every year in Japan. For soil levees, the fluctuation of the pore water pressure in the ground is a dominant parameter of their weakening. The multipoint monitoring of the pore water pressure in a levee allows us to grasp the risk of levee collapse directly. In this study, we developed a new ultrasmall-pore water pressure gauge (UPWP gauge) whose production cost and power consumption are lower and whose installation is easier than those of commercially available pore pressure gauges. We performed basic laboratory experiments for this UPWP gauge and pooling experiments for a full-scale-model levee and assessed their performance. Our results revealed that the errors of the values measured using the UPWP gauge in the laboratory test were within ±15 mm for the set water levels. Furthermore, in the verification of the full-scale test levee, the elevation process, and the maximum pore water pressures of the UPWP and the existing pore pressure gauges were in good agreement.