  We studied the eigenvalue problem of scalar fields in the (2+1)-dimensional
BTZ black hole spacetime. The Dirichlet boundary condition at infinity and the
Dirichlet or the Neumann boundary condition at the horizon are imposed.
Eigenvalues for normal modes are characterized by the principal quantum number
$(0<n)$ and the azimuthal quantum number $(-\infty<m< \infty)$. Effects to
eigenvalues of the black hole rotation and of the scalar field mass are studied
explicitly. Relation of the black hole rotation to the super-radiant
instability is discussed.
