To provide a stain preventing sheet by easy installation with which stains originated from excrement are prevented from being stuck to the inner wall surface of a toilet.

This stain preventing sheet 10 for a western style toilet composed of water soluble paper is in a shape foldable so as to be planar when not in use and is turned to a three-dimensional shape in an unfolded state at the time of use. The three-dimensional shape is provided with an inner wall surface protective part 11 having a shape capable of covering the inner wall surface on the inner side of a toilet body and a tongue piece part 13 for mounting projected and formed at an upper end edge in a mounting state of the inner wall surface protective part 11, which can be hooked to the upper surface of a toilet seat positioned at the upper part of the western style toilet. By such constitution, the sheet is piled up and kept in a small space before use, the stains in use are surely prevented when it is mounted, and even when the toilet inner wall is already stained, the stain is easily hidden. After the use, it is detached from the toilet and flushed to sewerage.