A lens unit LU in which a lens portion L is formed at least on either a light-receiving surface or a light-emitting surface of a lens holding plate P, wherein a difference in coefficients of linear expansion of the lens holding plate P and the lens portion L is used to suppress mis-focusing by causing changes in a paraxial image point position of the entire lens system due to changes in the surface shape of the lens portion L which accompany temperature changes, and changes in the paraxial image point position of the entire lens system due to changes in the refraction index to cancel each other out. Furthermore, deterioration of performance of the entire lens system caused by rising temperature is suppressed by satisfying a predetermined conditional expression.