Small specimens taken from their original surfaces have been used to measure surface admittance. This method shows good accuracy, but it requires a large number of specimens to measure the surface admittance of an entire surface. In this paper, we proposed a method to measure in-situ surface admittance by employing BEM-based acoustic holography. The method gives the surface admittance of the surface covered by a microphone array. The BEM-based acoustic holography was employed to reconstruct surface velocity and pressure. The ratio of the surface velocity to the surface pressure gives surface admittance. Simulations and experiments for interior and exterior sound fields verified the proposed method.