The small-hole sensor of Mitech portable surface roughness tester MR200, with totally length of 77mm, main body length of 46mm, guide-head bearer length of 19.6mm, main body diameter of Φ10mm, core-insert sleeve diameter of Φ7mm, guide-head height of 5mm, It is the optional configuration of the surface roughness tester, mainly used for measuring the inner surface roughness of the circular hole with a diameter larger than 5mm. And its other usage is the same as the standard sensor.