We describe near-infrared acousto-optic (AO) spectrometer ISEM (Infrared Spectrometer for ExoMars) developed for ExoMars 2020 space mission. The instrument goal is to investigate Martian surface and in particular to study mineralogical and, probably, petrographic composition of the uppermost regolith layer of the regolith by measuring reflected solar radiation in the near infrared spectral range. The instrument covers the wavelength range from 1.15 to 3.3 μm with the spectral resolution of ~25 cm-1 and has a circular field of view (FOW) of about 1°. The spectrometer consists of two parts: optical box and electronic box. The optical box is to be mounted on the top of the Martian rover’s mast providing better field of vision. Here we present the instrument description and optical design as well as the first results of laboratory calibrations and ground-based measurements of the Martian analogue samples.