To provide a waterproof pan structure of a shower core unit having no step at an entrance, having an opening part for level adjustment and maintenance operations, capable of preventing users from overturning in use, and manufacturable at low cost.

In this waterproof pan structure of the shower core unit, the dimensional ratio of its short side to its long side is 1:1.3 or larger. A drain port 3 is disposed near the short side center thereof. Seven or more drain sloped surfaces are formed in an area where a distance t from a long side wall panel installation face 2a is on the inner side by 30 mm or longer.