To miniaturize an X-ray detector without requiring a capacitor module by accumulating the current flowing on a signal electrode in a parallel flat capacitor formed with the signal electrode as charges.

Insulating layers 2 made of an insulating material are provided on both faces of a guard electrode 1 made of a conductive material, and electrodes 3 made of a copper foil are provided on the insulating layers 2 to form a double-faced signal electrode 10 with a laminated structure. The electrodes 3 are connected to a voltage converter via lead wires Ls, and the guard electrode 1 is connected to an earth bus LA via a lead wire La. The signal electrode 10 forms a parallel flat capacitor with the guard electrode 1, insulating layers 2, and electrodes 3. The current flowing on the electrodes 3 is accumulated in the parallel flat capacitor as charges, and a capacitor module can be omitted.