Ellipsoidal BGK model (ES-BGK) is a generalized version of the BGK model where the local Maxwellian in the relaxation operator of the BGK model is extended to an ellipsoidal Gaussian with a Prandtl parameter ν, so that the correct transport coefficients can be computed in the Navier-Stokes limit. In this work, we consider the existence and uniqueness of stationary solutions for the ES-BGK model in a slab imposed with the mixed boundary conditions. One of the key difficulties arise in the uniform control of the temperature tensor from below. In the non-critical case (−1/2 < ν < 1), we utilize the property that the temperature tensor is equivalent to the temperature in this range. In the critical case, (ν = −1/2), where such equivalence relation breaks down, we observe that the size of bulk velocity in x direction can be controlled by the discrepancy of boundary flux, which enables one to bound the temperature tensor from below.