To provide a method for constructing a wall with a ditch for draining rain water on the wall even by unskilled persons, and also to provide a plasterer trowel used for the same.

Guides 22, 23 are provided to forms 20, 21. The guides 22, 23 includes first and second depth guides 24, 25 which regulate the positions of bottoms 16, 17 in the depth direction of the drainage ditch, and first and second position guides 26, 27 which regulate the positions of the bottoms 16, 17 in the width direction of the drainage ditch. A user moves the trowel 10 for a plasterer along these guides 22, 23, forming the predetermined drainage ditch.