In an axial turbine wheel including a rotor disk and a plurality of turbine blades each having a base end retained by the rotor disk via a joint and extending radially from the rotor disk, a seal plate is received in outer and annular slots formed in the base ends of the turbine blades and rotor disk, respectively, and an annular shoulder is formed in the wall defining the inner annular slot on a side remote from the rotor disk, the annular shoulder thereby defining an inner narrow section and an outer wide section in the inner annular slot. Thus, the seal plate can be easily placed in position by first fitting the inner peripheral edge thereof in the inner narrow section of the inner annular slot, fitting the outer peripheral edge thereof in the outer annular slot until the inner peripheral edge clears the annular shoulder. Thereby, the outer and inner peripheral edges of the seal plate are received in the outer annular slot and outer wide section of the inner annular slot. The outer peripheral edge of the seal plate may abut the bottom of the outer annular slot so as to allow the seal plate to be firmly retained even when subjected to a large centrifugal force.