To prevent unnecessary mortar from flowing down into a hole in the middle of a block concurrently serving as a basic horizontal reinforcement.

In a tool for preventing mortar from flowing down, (A) a necessary part of the outer periphery of paper and the like is formed in a corrugated shape or the like, and when inserted into the groove of the block, a part having a corrugated shape or the like produces backstop effect by expanding force of the paper to prevent the tool from dropping out easily, and (B) as shown in Fig.7, two curved surfaces are formed by bending the tool so that a pair of the ends of one curved surface is brought into press-contact with the short side direction of the hole in the middle of the block and that the other curved surface is brought into press-contact with the transverse direction of the groove of the block, into which the horizontal reinforcement is inserted.