To substantially eliminate deterioration of a saturation characteristic of gamma rays.

A limitation is provided whereby the sum of areas of a plurality of holes 9 formed at a signal electrode 2 is set to be not more than 5% of the surface area of the signal electrode 2. When a distance W1 between adjacent holes 9 and 9 is set to be not smaller than twice a distance D between the signal electrode 2 and a compensation electrode 3, a leaking electric field of each hole can be considered independently. When a part where the hole 9 of the signal electrode 2 intersects a surface 2a, 2b is made a curved surface 9b, 9c, a sudden change of the electric field is eliminated.