To provide a water filling pipe structure of return underground water and a condensate well structure, capable of dispensing with cleaning operation, by preventing the occurrence of clogging by a floating suspended substance, bacteria and oxide of the dissolving iron content in a condensate well.

This water filling pipe structure pressurizes and returns pumped-up underground water in the ground; and is characterized in that a water filling pipe 3 has a strainer part 6 in a lower end part, and this strainer part 6 has a large number of water filling holes 10 formed at an interval in the peripheral direction and the longitudinal direction over a predetermined length range on a pipe wall of the water filling pipe itself.