To detect presence of a detect projecting in a passage accurately by setting the size of an internally moving spherical body smaller than the diameter of an opening and the nominal diameter of the passage but larger than the outlet diameter.

The diameter of a spherical body 10 is set smaller than the diameter A of an opening 3a at a tank section 3 and the nominal diameter C of the passage at a port 4 but larger than the outlet diameter B and tour spherical bodies 10 are located at respective port inlets. Attitude of an intake manifold 1 is then varied by driving a turning unit 14 so that the spherical body 10 is removed from the recess in a spherical body holding plate 11a and passed through the opening 3a and the port 4. If a projecting defect is not present in each port 4, the spherical body 10 stagnates in the vicinity of the outlet of the port 4 and detected by an insertion sensor. When four spherical bodies 10 are detected, a decision is made that a projecting defect is not present in each port 4. When three spherical bodies 10 or less are detected, a decision is made that a projecting defect is present in the port 4 of a sensor which does not detect the detect. After determining the presence of a projecting defect, the turning unit 14 is turned reversely and the spherical body 10 is returned back to the recess in the spherical body holding plate 11a.