The present invention provides a synthetic silica glass for an optical member in which not only a fast axis direction in an optical axis direction is controlled, and a birefringence in an off-axis direction is reduced, but a magnitude of a birefringence in the optical axis direction is controlled to an arbitrary value, such that an average value of a value BR cos2θxy defined from a birefringence BR and a fast axis direction θxy as measured from a parallel direction to the principal optical axis direction is defined as an average birefringence AveBR cos2θxy, and when a maximum value of a birefringence measured from a vertical direction to the principal optical axis direction of the optical member is defined as a maximum birefringence BRmax in an off-axis direction, the following expression (1-1) and expression (2-1) are established: −1.0≦AveBR cos2θxy