To correctly detect an eccentric volume at a tip end part of a mandrel in a horizontal extrusion press used for manufacturing an extrusion pipe.

Two directionally separated distance sensors 111, 112 are furnished at a given interval to a supporting cross-head 2 main ram 1 in an axial direction thereof. Each sensor measures a separated distance from a reference face of the cross-head 2 in vertical and horizontal directions. The eccentric volume at the tip end part of the mandrel is computed at a computing part 17 based on one set of two directionally separated distance signals R1, R2 obtained by simultaneous measurement with the two directionally separated distance sensors 111, 112 at 2 positions in an axial direction of the cross-head 2. A parallel variation volume from a press center line of the main ram 1, and a slanting angle against the press center line of the main ram 1 are obtained and the eccentric volume of the mandrel tip end can thus be obtained accurately.