A heat treatment method for synthetic quartz glass, said method comprising: heat treating a columnar synthetic quartz glass article having two opposing end faces (101; 102) and a lateral face covered with thermal insulator; wherein said thermal insulator has an end-face thermal insulator (21; 31; 22, 32) that covers said two end faces (102, 103), and a lateral face thermal insulator (23; 33) that covers said lateral face, said end-face (21; 31; 22, 32) and lateral face (102; 103) thermal insulators each having a respective type or thickness, wherein at least either said types or said thicknesses differ so as to afford different thermal insulation effects so as to regulate a birefringence fast axis direction of said synthetic quartz glass article either in tangential direction or in radial direction, whereby a regulated distribution of one of said fast axis directions is obtained if a mean value of the radial direction component of the birefringence size and a mean value of the tangential direction component of the birefringence size are compared with each other so as to obtain a ratio, and when the ratio is 2:1 or higher, the fast axis direction of said synthetic quartz glass article is defined as the component in said ratio which is 2 or higher, and whereby said mean values are obtained by measuring an angle (θ i) of the fast axis direction at several measurement points (a i) so as to obtain at each of said several measuring points (a i) the radial direction component of the birefringence size (Re i · cosθ i) as well as the tangential direction component of the birefringence size (Re i · sinθ i), and the mean value of the radial direction component of the birefringence size is expressed as follows: ∑ i = 1 n Re i ⋅ cos θ i n and the mean value of the tangential direction component of the birefringence size (Re i · sin θ i) is expressed as follows: ∑ i = 1 n Re i ⋅ sin θ i n heat treating a columnar synthetic quartz glass article having two opposing end faces (101; 102) and a lateral face covered with thermal insulator; wherein said thermal insulator has an end-face thermal insulator (21; 31; 22, 32) that covers said two end faces (102, 103), and a lateral face thermal insulator (23; 33) that covers said lateral face, said end-face (21; 31; 22, 32) and lateral face (102; 103) thermal insulators each having a respective type or thickness, wherein at least either said types or said thicknesses differ so as to afford different thermal insulation effects so as to regulate a birefringence fast axis direction of said synthetic quartz glass article either in tangential direction or in radial direction, whereby a regulated distribution of one of said fast axis directions is obtained if a mean value of the radial direction component of the birefringence size and a mean value of the tangential direction component of the birefringence size are compared with each other so as to obtain a ratio, and when the ratio is 2:1 or higher, the fast axis direction of said synthetic quartz glass article is defined as the component in said ratio which is 2 or higher, and whereby said mean values are obtained by measuring an angle (θ i) of the fast axis direction at several measurement points (a i) so as to obtain at each of said several measuring points (a i) the radial direction component of the birefringence size (Re i · cosθ i) as well as the tangential direction component of the birefringence size (Re i · sinθ i), and the mean value of the radial direction component of the birefringence size is expressed as follows: ∑ i = 1 n Re i ⋅ cos θ i n and the mean value of the tangential direction component of the birefringence size (Re i · sin θ i) is expressed as follows: ∑ i = 1 n Re i ⋅ sin θ i n