A navigation system for an acetabular cup, which guides an insertion orientation of the acetabular cup inserted into a pelvis during a total hip replacement surgery, includes: a pelvis position tracer which includes probes in contact with three particular points of the pelvis placed on an anterior pelvic plane and a first reference mechanism disposed to indicate a specific reference plane when the probes come in contact with the particular points; and a pelvis position indicator which is fixed to the pelvis, and includes a second reference mechanism that is adjustable to indicate a plane parallel to the specific reference plane indicated by the first reference mechanism, or to indicate a plane perpendicular thereto, or to indicate the both planes. Accordingly, an insertion orientation of an acetabular cup can be guided by using a reference mechanism having a simple structure, and the acetabular cup can be accurately guided regardless of changes in the patient's pelvic position during surgery, because a plane used in the insertion of the acetabular cup can be indicated continuously.