Cross sections of single and double electron detachment for negative heavy ion beams were investigated from the standpoint of negative-ion beam transport and application. The ion energy dependence of these cross sections was measured in the energy range from 5 to 50 keV. The measurement system, i.e., the negative ion implanter developed by the authors, consists of three chambers for beam collimation, collision and detection. In the detection chamber, an electrostatic deflector was provided to separate the interacted ion beam into negative and positive ion beams, which were independently measured. Argon and nitrogen gases were examined as target gases. Single and double electron detechment cross sections were obtained. Single electron detachment cross sections were 1.15 × 10 1̄5 cm 2 for the C - ion beam and 1.4 × 10 -15 cm 2 for the Si - in argon gas. In nitrogen gas, the cross sectio twice that size. In the energy range corresponding to a velocity of 10 5m/s, the cross section has almost a constant value. Double electron detachment has a cross section of about 10 -17-10 16 cm 2, and increases with the beam energy.