Abstract: A K-filter is the one attached to an apparatus for serial high-speed rotatography for photographing an object to be examined by use of X-rays. The filter comprises two K-containers (5a and 5b) in which a space is defined for fluidly containing a radiopaque fluid substance, and a radiotransparent region (2) formed between the two K-containers (5a and 5b). The K-containers (5a and 5b) are pivotally supported by a supporting member (1a) and are of the inversion type which allows the radiotransparent region sides of the K-containers to move. Therefore, when this apparatus is rotated at a high speed to more than 180.degree., it is possible to conduct filtering under any conditions such as an inclined posture, not the horizontal posture, under which photographing is started.