  In this study we investigate the bound states of the Hamiltonian describing a
quantum particle living on three dimensional straight strip of width $d$. We
impose the Neumann boundary condition on a disc window of radius $a$ and
Dirichlet boundary conditions on the remained part of the boundary of the
strip. We prove that such system exhibits discrete eigenvalues below the
essential spectrum for any $a>0$. We give also a numeric estimation of the
number of discrete eigenvalue as a function of $\displaystyle \frac{a}{d}$.
When $a$ tends to the infinity, the asymptotic of the eigenvalue is given.
