A reflective-type linear scale with a pitch of $1.67{mu}m$ used in an interferential scanning-type 2-DOF linear encoder is evaluated by the Fizeau interferometer with a measurement range of 100 mm. The 2-DOF linear encoder produces 2-axis position signals based on the interference between the X-directional positive and negative first-order diffracted beams from the linear scale. Firstly, the Z-directional flatness $e_Z$(x,y) of the linear scale is evaluated from the wavefront of the zero-order diffracted beam reflected from the linear scale. The linear scale is then tilted to align the axes of the first-order diffracted beams with that of the interferometer so that the X-directional period deviation eX(x,y)of the linear scale can be evaluated from the wavefronts of the X-directional positive and negative first--order diffracted beams. Finally, the Z-directional flatness $e_Z$(x,y) and X-directional period deviation $e_X$(x,y) were verified by comparing those with the nonlinear components of the 2-DOF linear encoder using the evaluated linear scale.