PURPOSE: To realize a detector which has a simple and strong structure and can use a simple electric signal processing circuit by stretching a linear anode and position reading electrodes in an airtight container with a radioactive ray incidence window in the preset mode.

CONSTITUTION: An anode wire 2 is stretched along an incidence window 4 in an airtight container 1 having the radioactive ray incidence window 4 made of beryllium and concurrently serving as a cathode, and position reading electrodes 3a, 3b are stretched. The electrodes 3a, 3b are stretched in the directions so that their distances from the anode wire 2 are increased or decreased. The electrode 3a is gradually separated from the anode wire 2 from one end of the anode wire 2 toward the other end, and the electrode 3b gradually approaches the anode wire 2. The position coordinates X of the electron avalanche generated at the point A on the anode wire 2 by the incident radioactive rays are calculated based on the charges induced on the electrodes 3a, 3b respectively and the distances between the electrodes 3a, 3b and the anode wire 2 respectively.