Compressed air is delivered through tube 1 into membrane-type pressure stabilizer 2 and further, through internally tapered glass indicator column 3, containing lihghtweight float 4, to measuring head 5. The float is held in the suspended state by the flow of air. Depending upon clearance a between the end face of measuring head 5 an the surface of work d being checked, the flow of air and, consequently, its velocity are varied. Float 4 rises and lowers in column 3 until equilibrium is reached between the action of gravity and the air flow on the float. The scale, on which a reading is taken at the upper edge of the float, can be graduated in units showing the deviation of the work size from the basic value.