A filament for an X-ray tube has a varied wire diameter but has a constant coil outside diameter to obtain a good uniformity of the longitudinal temperature distribution of the filament. The filament has a wire diameter d which is gradually reduced from the longitudinal central region to the longitudinal ends while the coil outside diameter D is fixed along the longitudinal direction. The wire is polished at only the inside of the coil to reduce the wire diameter. In order to make the longitudinal temperature distribution uniform as far as possible, the difference Δd between the wire diameter dmax at the longitudinal central region and the wire diameter dmin at the longitudinal ends should satisfy the following limitation: Δd/dmax=0.041 to 0.145.