To provide a security instrument which can prevent an intruder from feeling suspicious of it.

A security sheet 101 includes a base layer sheet and an adhesive sheet. The upper surface of the base layer sheet is provided with a design pattern. The adhesive sheet is composed of a material with translucency. The adhesive sheet is laminated on the upper surface of the base layer sheet. In a predetermined security zone 202, the security sheet 101 and an installation object 301 having the same pattern as the design pattern on its upper surface are installed in turns depending on a time period.