  We have mapped the Sunyaev-Zel'dovich decrement (hereafter SZ) in the
direction of the most luminous X-ray cluster known to date, RXJ1347-1145, at
z=0.451. This has been achieved with an angular resolution of about 23'' using
the Diabolo photometer running on the IRAM 30 meter radio telescope. We present
here a map of the cluster central region at 2.1mm. The Comptonization parameter
towards the cluster center, $\yc=(12.7^{+2.9}_{-3.1})\times 10^{-4}$,
corresponds to the deepest SZ decrement ever observed. Using the gas density
distribution derived from X-ray data, this measurement implies a gas
temperature $\te=16.2 \pm 3.8$ keV. The resulting total mass of the cluster is,
under hydrostatic equilibrium, $M(r<1 Mpc)=(1.0 \pm 0.3) \times 10^{15}
M_\odot$ for a corresponding gas fraction $f_{gas}(r<1 Mpc)=(19.5 \pm 5.8)%$.
