We apply intracavity absorption spectroscopy for measurements of the absorption spectrum of formaldehyde, CH2O, from 6230 to 6420 cm−1, of which only a small fraction (6351–6362 cm−1) has been recorded elsewhere. The measurements are performed in the cavity of a broadband Er3+-doped fiber laser, with a sensitivity corresponding to the effective absorption path length of 45 m and a spectral resolution of 0.1 cm−1. The noise-equivalent detection limit of CH2O achieved with the strongest absorption line at 6252.64 cm−1 is estimated to be 5 ppm. High tolerance to broadband losses and the accessible time resolution of 50 µs make it possible to apply this detection system for time-resolved monitoring of CH2O together with other molecules in harsh combustion environments, e.g., in combustion engines.