This paper deals with the stress concentration problems of an elastic strip with a pair of central notches and two pairs of supplemental notches under tension. The stress mitigations due to interference effects of stress concentrations by supplemental notches are investigated. Considerations are made for the influences of the distances between the central notches and the supplemental notches and the radii of the notches on the stress mitigations. The principle of the method of solution is to distribute body forces in the interior of a strip with no notch so as to satisfy the boundary conditions at the notches. For this purpose, we apply Green's functions for body force problems of a strip. The method of solution may be regarded as a kind of so-called indirect boundary element method with fictitious boundary.