Testable substrate and a testing method

A testable substrate for mounting an optical component, comprising:

• a base member having a plurality of optical component mounting areas in which light emitters and light detectors for detecting lights emitted from the light emitters are to be respectively mounted;
• a first insulating layer formed on the base member;
• a second insulating layer formed directly on the first insulating layer;
• two kinds of conductive lines connectable with opposite electrode terminals of light emitters mounted in the respective optical component mounting areas;
• and
• two kinds of conductive lines connectable with opposite electrode terminals of light detectors mounted in the respective optical component mounting areas;
• wherein at least one of the two kinds of conductive lines connectable with one light emitters and light detectors are laid on the first insulating layer, at least one of the two kinds of conductive lines connectable with the other of light emitters and light detectors are laid on the second insulating layer.