tunnel current detector 21 supplies servo circuit 13 with a signal representing the increase in the tunnel current which has resulted from the deposition of the film on the processed surface of the sample and has been actually detected servo circuit 13 feeds the signal back to piezoelectric member 11 thus expanding member 11 such that cantilever 14 is moved away from base 10 and set apart therefrom by the predetermined distance this servo system is similar to that employed in most stms since the distance between base 10 and probe 18 is maintained constant the tunnel current can be repeatedly detected with high efficiency the weight of the film deposited on the processed surface of the sample is computed from the integrated changes in the tunnel current displacement w of cantilever 14 can be determined as follows w pl 3 3ei wherein the young 39 s modulus of cantilever 14 is represented by e the sectional secondary moment of cantilever 14 by i load applied to the free end of cantilever 14 by p and the length of cantilever 14 by l providing that the width of cantilever 14 is denoted by b and the thickness thereof by h i bh3 12 therefore w 4 e u00b7 l h 3 u00b7 p b in a case where cantilever 14 is made of al having a thickness h of 1 u03bcm width b of 0 1 mm and length l of 0 5 mm and chromium having a density of 7 20 g cm 3 and.