PURPOSE: To provide an automatic variation apparatus for varying valve overlaps and valve opening cross-sections during engine operation in two sets of pure double-efficient shaft valves used in an internal combustion engine or other engines having a dynamic valve mechanism.

CONSTITUTION: The apparatus described comprises a shaft member 2 having a spiral threaded groove 41 at one end, an non-rotatable shaft casing 34 outer- fitted to engage with the threaded groove 41, and a motor reducer 42 pressing the shaft member 2 in the longitudinal axial direction via a spindle 44. Valve overlaps and valve opening cross-sections are varied when the shaft member 2 is pressed.