  We experimentally demonstrate the occurrence of negative absolute resistance
(NAR) up to about $-1\Omega$ in response to an externally applied dc current
for a shunted Nb-Al/AlO$_x$-Nb Josephson junction, exposed to a microwave
current at frequencies in the GHz range. The realization (or not) of NAR
depends crucially on the amplitude of the applied microwave current.
Theoretically, the system is described by means of the resistively and
capacitively shunted junction model in terms of a moderately damped, classical
Brownian particle dynamics in a one-dimensional potential. We find excellent
agreement of the experimental results with numerical simulations of the model.
