A control unit for a work machine that allows automatically obtaining optimal operability even when the weight of a work arm in the work machine is changed. In a work machine where at least a part of a work arm to be operated by a fluid pressure actuator is provided so as to be replaceable, a control unit for a work machine includes: a pilot-operated control valve that controls the fluid pressure actuator; a proportional solenoid valve that pilot-controls the pilot-operated control valve by a pilot control pressure according to an electrical signal corresponding to a manual operation amount; a measuring means that measures a weight of at least the part of the work arm; and a controller that converts characteristics between the manual operation amount and the pilot control pressure of the proportional solenoid valve to characteristics according to the weight of the work arm measured by the measuring means.