In this work, we use long-slit spectroscopy observations of a transit event of the close-in orbiting planet WASP-43b (Msubp=2.034 Msub Jup, Rsubp=1.036 RsubJup in an effort to detect its atmosphere. We used the Gran Telescopio Canarias (GTC) instrument OSIRIS to obtain long-slit spectra in the optical range 520-1040 nm of the planetary host star WASP-43 and of a reference star during a full primary transit event and four partial transit observations. We measure a mean planet-to-star radius ratio in the white light curve of 0.15988sub -0 .00145sup +0 .00133. Using broadband filters, we detect the color signature of WASP-43. We present a tentative detection in the planet-to-star radius ratio around the Na I doublet (lambda 588.9, 589.5 nm) when compared to the nearby continuum at the 2.9sigma level. As previous results, this hints to the orbital decay of this planet although a timing analysis over several years needs to be made to confirm this.