Using the time domain formulation of the theory of the tunnel junction, we investigated Shapiro steps by digital simulation, under the condition of a constant current source given by

. The integral kernels for the Josephson and the quasiparticle current were computed assuming a nonzero pair breaking parameter

, and T = 0K. We obtained I
rfdependence of the zeroth and first Shapiro steps, I
0and I
1, and the frequency dependence of

, the maximum of the first Shapiro step as a function of I
rf, for a few values of the junction capacitance. we found the following results. (1)

(I
c:the critical current), showed the Riedel peak at

, where ω
gis the gap frequency 4 Δ/h. (2) For

agrees well with that for the constant voltage bias. (3) As the frequency becomes smaller below

is severely depressed compared to that for the constant voltage bias.