We report on the development of an optical instrument based on LED-IBBCEAS for simultaneous measurements of nitrous acid (HONO) and nitrogen dioxide (NO2) in ambient air. The light emitting from the LED centered at 365 nm was directly focused into the cavity formed with two high reflectivity mirrors, separated by a distance of 1.76 m. The light output of the cavity was received with a portable spectrometer. The mirror reflectivity was calibrated by absorption spectra of NO2 and O2-O2. In the spectral range of 353～376 nm, the maximum mirror reflectivity was found to be 0.999 17. Detection limits (1σ) of 0.6 ppbv for HONO and 1.8 ppbv for NO2 were achieved with an acquisition time of 120 s. In order to test the accuracy of measured results by present setup, concentrations of NO2 were recorded during continuous 56 hours and compared with data from a NOX analyzer equipped with a blue light converter. The least-square fit lines give gradients of 1.09 and respective intercepts of 3.45, with a linear correction factor of 0.89. The concentrations of HONO and NO2 in indoor air were monitored, the concentrations of HONO varied from near 0 to 5.3 ppbv in 24 hours, the averaged concentration was 1.8 ppbv, and the concentrations of NO2 varied from 5 to 51 ppbv at the same time, the averaged concentration was 21.9 ppbv.