Measuring method for CTE (coefficient of thermal expansion) of honeycomb sandwich panels with face sheets made of polymer matrix composite was studied. At first, the optimum size of specimens for an accurate measurement was calculated by using the finite element method. Then, based on the results of the calculation, a CTE measuring system was developed utilizing photoelectric displacement sensors. The accuracy of the system was evaluated employing zero-expansive glass and carbon fiber reinforced plastic as a reference material whose CTE is estimated with an accuracy of 0.01×10^-6/K. As the result, the system was found to have an accuracy of 0.1×10^-6/K or better. Furthermore, measuring error of commercially available delatometer that employs a smaller size specimen was studied. The finite element analysis and the experimental study indicated that the error in that conventional measurement is approximately 0.2×10^-6/K.