© 2015 The Japan Society for Aeronautical and Space Sciences. A new method of strengthening adhesive bonding structures, introducing a slit near the bonding layer, is proposed in order to solve a problem affecting composite pressure vessels; that is, delamination of the bonding layer between the composite and metal, especially around a mouthpiece. The effect of the slit is studied on the basis of an analytical solution, and evaluated using actual double cantilever beam experiments and finite element method calculations. An example of composite pressure vessel design is presented to illustrate the effect of the slit.