To perform a leakage diagnosis of a control valve easily and quantitatively regardless of the kind of gland packing.

A fluid leakage diagnostic section 2-3 is provided to the positioner 2 of the leakage diagnostic system. The fluid leakage diagnostic section 2-3 sends a drive command to an opening control part 2-1, and makes a valve body 1-2 perform reciprocating motion a predetermined number of times. Then it samples a frictional force F which is determined from an axial load W detected by a sealing force F axial force sensor S2 during this time and fluid pressure P detected by a pressure sensor S3 as a pair with the same timing. Then the fluid leakage diagnostic section 2-3 determines Mahalanobis distance Di'Di' to Dm' which indicate the alienation of each of the sampled data groups Fi', Pi' from the barycenters Fav, Pav of the data groups of the sealing force F and the fluid pressure P which have been created by a threshold-value creation means. Then the fluid leakage diagnostic section 2-3 determines whether the fluid is leaking from a gland packing 1-4 based on the comparison result between the average value Dav' of the Mahalanobis distances D1' to Dm' and a threshold value Dth in a memory M1.