A parallelism fixing device applied to dual-shaft system for fixing the parallelism of the rotary shafts and facilitating the assembling process. The parallelism fixing device includes a first rotary shaft, a second rotary shaft and a fixing unit, which are assembled with each other. Each of the first and second rotary shafts has a fixed section mounted on an electronic apparatus, a pivoted section assembled with a torque module and a middle section positioned between the fixed section and the pivoted section. The fixing unit is assembled with the middle sections. The fixing unit has a substantially 8-shaped cross section and includes a first chamber, a second chamber and a belly section in connection with the first and second chambers. The middle sections of the first and second rotary shafts are respectively fixedly assembled in the first and second chambers.