We have conducted an LTE abundance analysis for SAO 40039, a warm post-AGB star whose spectrum is known to show surprisingly strong He i lines for its effective temperature and has been suspected of being H-deficient and He-rich. High-resolution optical spectra are analyzed using a family of model atmospheres with different He/H ratios. Atmospheric parameters are estimated from the ionization equilibrium set by neutral and singly ionized species of Fe and Mg, the excitation of Fe i and Fe ii lines, and the wings of the Paschen lines. On the assumption that the He i lines are of photospheric and not chromospheric origin, a He/H ratio of approximately unity is found by imposing the condition that the adopted He/H ratio of the model atmosphere must equal the ratio derived from the observed He i triplet lines at 5876, 4471 and 4713Å, and singlet lines at 4922 and 5015Å. Using the model with the best-fitting atmospheric parameters for this He/H ratio, SAO 40039 is confirmed to exhibit mild dust-gas depletion, i.e., the star has an atmosphere deficient in elements of high condensation temperature. The star appears to be moderately metal-deficient with [Fe/H]=−0.4 dex. But the star's intrinsic metallicty as estimated from Na, S and Zn, elements of a low condensation temperature, is [Fe/H] o 1 =≃ −0.2. The star is enriched in N and perhaps O too, changes reflecting the star's AGB past and the event that led to He enrichment.