We present a new diagnostic diagram based on ISOCAM spectra (5-16 mu m) to
distinguish the emission induced by the active galactic nucleus (AGN) from
that associated with the star formation activity. This diagnostic based on
the mid-IR continuum and the Unidentified Infrared Band (UIB) intensity all
ows us to estimate the relative importance of the three components (HII reg
ions, diffuse/photo-dissociation regions (PDRs), and AGN) contained in gala
xy mid-infrared emission. In AGN spectra, we confirm the absence of UIBs pr
esumably photodissociated by the X-UV radiation field. In addition, a non-n
egligible continuum below 9 mu m commonly associated with emission from hot
dust is present in AGNs. A diagnostic diagram derived from these two resul
ts can be used for detecting obscured AGNs embedded in a large concentratio
n of dust.