In order to properly evaluate the effect of tensioning on a circular saw, it is essential to use the change in the natural frequency of the saw blade when tensioning. This paper presents a spatial filter called a modal filter to easily measure the natural frequencies for only the modes necessary for the evaluation of the tensioning effect. The modes necessary for the evaluation are indicated based on the backward traveling wave frequencies of a rotating circular saw, which are calculated using the finite element method. The modal filter is designed based on the mode shapes of a disk. The examples show that the natural frequencies for the modes with nodal diameter numbers 0, 1 and 2 are necessary for the evaluation of the tensioning effect. The information on the nodal diameters of a disk enables us to easily design proper sensor locations for the modal filter for a disk such as a circular saw. The modal filter designed extracts separately the three modes necessary for the evaluation.