A robust absorption spectrometer using the off-axis integrated cavity output spectroscopy (ICOS) technique in a passive cavity is presented. The observed sensitivity, conceptually the detection threshold for the absorption cross section \left({\mathrm{cm}}^2\right) multiplied by the concentration \left({\mathrm{cm}}^{-3}\right) and normalized by the averaging time, is measured to be 1.9\times {10}^{-12}\left(1/\mathrm{cm}\sqrt{\mathrm{Hz}}\right) . This high sensitivity arises from using the optical cavity to amplify the observed path length in the spectrometer while avoiding cavity resonances by careful design of the spot pattern within the cavity. The instrument is ideally suited for routine monitoring of trace gases in the near-infrared region. A spectrum showing ambient carbon monoxide at 1.57\mathrm{\mu m} is presented.