A coiled filament for an X-ray tube has a varied coil pitch to obtain a good uniformity of the longitudinal temperature distribution. The filament has a central region including plural turns having a same coil pitch, and end regions which include plural turns each of which has a coil pitch smaller than the coil pitch of the central region. The coil pitches of the plural turns of the end regions are reduced one by one by a same variation from a turn close to the central region toward an outermost turn. A value of .DELTA.p/p is within a range of 0.015 to 0.1 and k/n is within a range of 0.3 to 0.8, where p is the coil pitch of the central region, .DELTA.p is the coil pitch variation of the end regions, n is a total number of turns of the filament, and k is a sum of numbers of turns of the end regions. The k/n preferably satisfies the following equation: k/n=0.72-4.66(.DELTA.p/p).+-.0.12.