Uncoupling device for a drill string, comprising in a longitudinal direction: - a first structure which is to be fixed to a tube of a drill string, - a second structure which is to hold a cutting tool for drilling, said second structure being movable in translation relative to the first structure in the longitudinal direction, - a thruster assembly between said first structure and said second structure, said thruster assembly being suitable for exerting a thrusting force on said second structure in order to separate said second structure from said first structure, said thrusting force being substantially constant for every position of the second structure relative to the first structure, said uncoupling device being characterised in that the second structure is guided in translation relative to the first structure in the longitudinal direction, and the thruster assembly comprises at least: - a counterbalancing mechanism comprising a resilient member (21, 22) and an output arm, the resilient member (21, 22) producing a resilient force proportional to a compression (x) of the resilient member, the output arm being connected to said resilient member (21, 22) and being capable of pivoting about an output shaft (29 1), and - a linking means between said output arm and said second structure for applying said thrusting force to said second structure in said longitudinal direction, said linking means comprising a wheel (31 2) which is articulated for rotation relative to the output arm, at a distance from said output shaft (29 1), and rests against a linking surface which is connected to the second structure and is substantially perpendicular to the longitudinal direction. - a first structure which is to be fixed to a tube of a drill string, - a second structure which is to hold a cutting tool for drilling, said second structure being movable in translation relative to the first structure in the longitudinal direction, - a thruster assembly between said first structure and said second structure, said thruster assembly being suitable for exerting a thrusting force on said second structure in order to separate said second structure from said first structure, said thrusting force being substantially constant for every position of the second structure relative to the first structure, said uncoupling device being characterised in that the second structure is guided in translation relative to the first structure in the longitudinal direction, and the thruster assembly comprises at least: - a counterbalancing mechanism comprising a resilient member (21, 22) and an output arm, the resilient member (21, 22) producing a resilient force proportional to a compression (x) of the resilient member, the output arm being connected to said resilient member (21, 22) and being capable of pivoting about an output shaft (29 1), and - a linking means between said output arm and said second structure for applying said thrusting force to said second structure in said longitudinal direction, said linking means comprising a wheel (31 2) which is articulated for rotation relative to the output arm, at a distance from said output shaft (29 1), and rests against a linking surface which is connected to the second structure and is substantially perpendicular to the longitudinal direction.