The polarimeter built for the high resolution spectrograph (SARG) of the alto-azimuthal Telescopio Nazionale Galileo is presented. This double-beam instrument, able to take into account time independent (instrumental) and time dependent (sky transparence) sensitivity, is based on a Fresnel prism (λ/2) and K-prism (λ/4) which gives an almost constant retard along the very large wavelength interval covered with new spectrographs: SARG covers the 370 - 1020 nm range and more than 300 nm in a single exposure. The two flat metallic mirrors, which are necessary to feed the spectrograph, and the alto-azimuthal mounting of the telescope are responsible of an instrumental polarisation depending on the sky position of the target. A modelling of the instrumental polarisation and a hardware correction of the sky rotation are performed to measure the polarisation across stellar-like object at R=115,000 resolution.