A big spectrometer magnet, three hundred tons, has been successfully designed and measured at Saclay. Its main features are a fixed working induction of 3 Tesla and a total flux of 10 Weber. The coils are conventional and for the yoke we use a scrapped cyclotron. The spectrometer is of an Elbek type and its optic characteristics are a momentum analysis range from 0.6 to 1.4 GeV/c, with a 5.10-4resolution. The angular analysis lies between -5° and 65° around the target. The magnetic study and optimisation of the magnet has been done with Poisson, bidimensional code and Ledi which is a simplified three dimensional code developed for fully saturated iron magnets.