PURPOSE: To efficiently cool the electronic circuit parts of all radiation detection units by providing cooling liquid percolation ducts where cooling liquid flows through heat conduction plates on the reverse surfaces of the electronic circuit parts of all radiation detection units.

CONSTITUTION: Heat conduction plates 4 are fitted in two-dimensional array on the reverse surfaces of the electronic circuit chips 3 of radiation detection units and cooling material percolation ducts 6 are arranged on the downstream sides of the heat conduction plates 4. In this constitution, the cooling water is made to flow in the ducts 6 and respective chips are cooled almost uniformly through the heat conduction plates 4. Further, an X-ray detecting element 1 and a chip are connected by a bump 2 made of solder.