  We study quasinormal modes (QNMs) of massive Klein-Gordon fields in static
Ba\~{n}ados-Teitelboim-Zanelli (BTZ) black holes in terms of ladder operators
constructed from spacetime conformal symmetries. Because the BTZ spacetime is
locally isometric to the three-dimensional anti-de Sitter spacetime, ladder
operators, which map a solution of the massive Klein-Gordon equation into that
with different mass squared, can be constructed from spacetime conformal
symmetries. In this paper, we apply the ladder operators to the QNMs of the
Klein-Gordon equations in the BTZ spacetime. We demonstrate that the ladder
operators can change indices of QNM overtones, and all overtone modes can be
generated from a fundamental mode when we impose the Dirichlet or Neumann
boundary condition at infinity. We also discuss the case with the Robin
boundary condition.
