This paper describes a scanning multi-probe measurement system for local alignment of linac components. The system consists of two probe-units, each having three displacement probes. The two probe-units, which are placed on the two sides of the cylindrical linac components, are moved by a scanning stage with a scanning range of 5 m to simultaneously scan the two opposed straightness profiles of the linac cylinders. A differential output calculated from the probe outputs in each probe-unit cancels the influence of error motions of the scanning stage, and a double ntegration of the differential output gives the straightness profile. The difference between the unknown zero-values of the probes in each probe-unit of zero-difference, which introduces a parabolic error term in the profile evaluation result, is calculated and compensated for by a zero-adjustment method so that accurate straightness profiles of the linac cylinders can be obtained. The effectiveness of the measuring system is confirmed by experimental results.