To provide an air cylinder type foundation strength investigation machine which can investigate accurate foundation strength by inhibiting a penetration body from rapidly self-settling into a soft layer of ground.

This swedish-weight-sounding air-cylinder type foundation strength investigation machine 10 digitally controls a variation in the self-settlement of the penetration body by means of an air cylinder 44. The foundation strength investigation machine 10 includes: a loading elevating platform 23 which comprises the penetration body 30 perpendicularly thrusting a leading end of a penetration rod 31 into the ground, a rotary device for rotating the penetration body, and a weighting means for weighting the penetration rod; an air cylinder type elevating mechanism for lifting/lowering the loading elevating platform; and a measuring device for measuring the penetration depth of the penetration rod and the number of the rotations of the penetration rod. The weighting means makes a direction of an upward thrust by air pressure of the air cylinder face a direction of a downward thrust by an applied weight load 22, so that a difference between the thrusts can make the penetration body self-settle and ascend/descend. A constitution, which performs sequence control of a digital regulator, is employed for the control of the air pressure.