A sample plate 3 with a sample 4 placed thereon is initially set on a stage 2, and a visual image of the sample is taken with a CCD camera 14. This image is stored in an image data memory 23. Then, an operator removes the sample plate 3, sprays a matrix for a MALDI process onto the sample 4 and replaces the plate onto the stage 2. After that, when a predetermined operation is made, a clear image of the sample taken before the application of the matrix is shown on a display unit 24. On this image, the operator specifies a point or area for the analysis. The sample 4 may have been displaced due to the removal and replacement of the plate 3. Accordingly, an image analyzer 44 calculates the direction and magnitude of the displacement, for example, by recognizing the position of the markings provided on the sample plate 3. A displacement corrector 42 computes coordinate values in which the displacement is corrected. Thus, even if a displacement occurs, the mass analysis can be accurately performed on the point or area of the actual sample as specified on the clear visual image taken before the application of the matrix.