PURPOSE: To judge whether impurities are organic substance or metallic substance by attaching opposite electrodes to both sides of an organic insulating layer and by measuring electrostatic capacity between the opposite electrodes to estimate impurities contained in the organic insulating layer.

CONSTITUTION: A first electrode 32 is brought into contact with a part opposite to a first conductor thin film layer 12 on a surface of an organic insulating layer 14. A second electrode 34 is inserted to a via hole 18 of a ceramic substrate 10 to be brought into contact with the first conductor thin film layer 12. Thereby, the first electrode 32 becomes one counter electrode and the second electrode 34 becomes the other counter electrode together with the first conductor thin film layer 12. Then, a current is made to flow between the first electrode 32 and the second electrode 34 to measure electrostatic capacity. When impurities 20 are metallic substance, electrostatic capacity is considerably large and the impurities can be distinguished clearly from those of organic substance. Therefore, it is possible to judge whether the impurities 20 are organic substance or metallic substance by setting an allowable range of electrostatic capacity based on known dielectric constant of an organic insulating layer 14 and by determining whether a measured value is in the allowable range or not.