An injector for radiation testing (1, 1A to 1C) equipped with manual operation type syringe (3, 3A, 3B or 3C) for manually infusing materials (9, 9B, 9C) to a patient (11) under control has an impaction condition detecting means (4, 12, 4A, 21, 17C and 31) for detecting the impaction conditions of a container (31) filled up with the materials (9, 9B, 9C) to be infused. In order to detect the impaction conditions, either directly measuring the volume of materials (9, 9B, 9C) occupied within the container (31) or indirectly measuring the position of the constituent part (5) of the injector (1, 1A to 1C) is possible and the position of the constituent part is correlated with the occupied volume.