  The ac Josephson effect in a ferromagnetic Josephson junction, which is
composed of two superconductors separated by a ferromagnetic metal (FM), is
studied by a tunneling Hamiltonian and Green's function method. We obtain two
types of superconducting phase dependent current, i.e., Josephson current and
quasiparticle-pair-interference current (QPIC). These currents change their
signs with thickness of the FM layer due to the 0-$\pi$ transition
characteristic to the ferromagnetic Josephson junction. As a function of
applied voltage, the Josephson critical current shows a logarithmic divergence
called the Riedel peak at the gap voltage, while the QPIC shows a discontinuous
jump. The Riedel peak reverses due to the 0-$\pi$ transition and disappears
near the 0-$\pi$ transition point. The discontinuous jump in the QPIC also
represents similar behaviors to the Riedel peak. These results are in contrast
to the conventional ones.
