  We present results of ultra-deep ISOCAM observations through a cluster-lens
at 7 and 15 micron with the Infrared Space Observatory (ISO) satellite. These
observations reveal a large number of luminous Mid-Infrared (MIR) sources.
Cross-identification in the optical and Near-Infrared (NIR) wavebands shows
that about half of the 7 micron sources are cluster galaxies. The other 7
micron and almost all 15 micron sources are identified as lensed distant
galaxies. Thanks to the gravitational amplification they constitute the
faintest MIR detected sources, allowing us to extend the number counts in both
the 7 and 15 micron bands. In particular, we find that the 15 micron counts
have a steep slope alpha_15 = -1.5 +/- 0.3 and are large, with N_15 (>30
microJy}) = 13 +/- 5 per square arcmin. These numbers rule out non-evolutionary
models and favour very strong evolution. Down to our counts limit, we found
that the resolved 7 and 15 microns background radiation intensity is
respectively (2 +/-0.5) 10^(-9) and (5 +/-1) 10^(-9) W m^(-2) sr^(-1).
