Context. GJ 1214 b is one of the few known transiting super-Earth-sized exoplanets with a measured mass and radius. It orbits an M-dwarf, only 14.55? pc away, making it a favorable candidate for follow-up studies. However, the composition of GJ? 1214? b's mysterious atmosphere has yet to be fully unveiled. Aims. Our goal is to distinguish between the various proposed atmospheric models to explain the properties of GJ? 1214? b: hydrogen-rich or hydrogen-He mix, or a heavy molecular weight atmosphere with reflecting high clouds, as latest studies have suggested. Methods. Wavelength-dependent planetary radii measurements from the transit depths in the optical/NIR are the best tool to investigate the atmosphere of GJ? 1214? b. We present here (i) photometric transit observations with a narrow-band filter centered on 2.14? μm and a broad-band I-Bessel filter centered on 0.8665? μm, and (ii) transmission spectroscopy in the H and K atmospheric windows that cover three transits. The photometric and spectrophotometric time series obtained were analyzed with MCMC simulations to measure the planetary radii as a function of wavelength. We determined radii ratios of 0.1173-0.0024+0.0022 for I-Bessel and 0.11735-0.00076+0.00072 at 2.14? μm. Results. Our measurements indicate a flat transmission spectrum, in agreement with the last atmospheric models that favor featureless spectra with clouds and high molecular weight compositions. © ESO, 2014.