Deformation of the spot of an off-axis Gaussian beam after passing through a lens-like glass fiber (SELFOC®) with the fourth-order aberration is investigated by using first-order perturbation theory and numerical calculation. A method for determining the coefficient of the fourth-order aberration is proposed. By this method, it is found that a SELFOC sample (0.5 mm in diameter and 150 mm in length) has a coefficient

of about 1.1, if the dielectric constant in the meridional plane of the sample is expressed as

, where

is the distance from the axis of the medium and

is a positive constant.