To provide an undergroundwater level lowering apparatus which is lightweight, equipped with a decompressing means free from an influence of an undergroundwater level, simple and economically advantageous, and applicable to execution of a vacuum deep well construction method.

The undergroundwater level lowering apparatus 1 is formed of a well casing pipe 3 in which a water flow section 4 is formed, a pumping means 5, a decompressing means 6, a water level measuring pipe 7, and a filter layer 8. The decompressing means 6 is formed of a vacuum pump 18, and a suction pipe 19 which has an intake port 19a at a lower edge thereof located under the groundwater level inside 3b of the well casing pipe 3, in an inundated state, and functions to decompress the inside 3b of the well casing pipe 3.