We first consider a problem on tension of a plate with cracks; the plate is reinforced by a stringer and cracks are inclined with respect to the stringer at various angles. We present the results of model solution of this problem for plates manufactured from optically sensitive material reinforced by a stringer placed through the center of plates; the stringer is a duralumin rod of rectangular cross section; cracks are formed in advance such that they are inclined with respect to the stringer. We determine stress fields near the tips of the cracks on the bases of the polarization-optical method (photographs of zones of interference). We present the results for stresses near the crack tips next to the stringer in the form of quotient of the stress intensity factor for the plate reinforced by the stringer to the stress intensity factor for the plate with no reinforcement. We analyze how the stringer and the angle of inclination of a crack with respect to the stringer affect the value of this quotient. It is established that when tips of inclined cracks are at the constant distance from the stringer, the value of this quotient grows as the angle of inclination decreases. It is known that the usage of a stringer leads to the strengthening of a plate with cracks orthogonal to the stringer. Our results confirm this fact. At that, the value of the quotient mentioned decreases as the tip of the crack approaches the stringer.