PURPOSE: To provide an external wall structure which prevents the humid air under a floor from flowing inside a wall body and the construction processes are less and the ventilation efficiency is high, by providing a ventilation pass under the floor separately from a ventilation pass in the wall body.

CONSTITUTION: The external structure is provided with a sill 2 having a ventilation opening 3 under the floor, connected to the space 5 under the floor, on the upper part of the foundation 1. And a blind plate 19 is provided, which is connected to the external wall 12 provided with a ventilation layer 16 and extended to at least the lower end of the ventilation opening 3 under the floor, to form a ventilation space B communicating with the ventilation layer 16 of the external wall 12 between the blind plate and the foundation 1. A ventilation guide member 26 covering the ventilation opening 3 in the ventilation space B and having a ventilation opening 26a only at the lower part, is provided in correspondence with the ventilation opening under the floor between the blind plate 19 and the sill 2.