A radio-active radiation counter is provided with a counter chamber having a diaphragm-less open window, and two collecting electodes are disposed in said chamber and separated from each other. A gas to be ionized is injected in a pulse into the chamber intermittently to create a gas cloud for a short duration in the space where the electrodes are disposed, and a voltage pulse is applied across the electrodes during the presence of said gas cloud between the electrodes to operate the counter in order to measure the radiation coming into the chamber during the voltage being applied to the electrodes. Because of the absence of a diaphragm in the ray entrance window of the counter chamber, the absorption of radiation energy in a diaphragm is avoided and thereby it is possible to measure radiations of energy less than 1 Kev.