A posture computing apparatus for a work machine includes a detection apparatus that is provided to the work machine and detects angular velocity and acceleration; a first posture angle computing unit that is provided to the detection apparatus and obtains a posture angle of the work machine from the angular velocity and the acceleration detected by the detection apparatus; a low-pass filter that allows the posture angle obtained by the first posture angle computing unit to pass therethrough to output the posture angle as a first posture angle; a second posture angle computing unit that outputs, as a second posture angle, a posture angle obtained from the angular velocity and the acceleration detected by the detection apparatus; and a selecting unit that outputs the first posture angle and the second posture angle in a switching manner, based on information about a change in an angle of the work machine.