To improve the temperature stability of an optoelectronic element against the fluctuation of the ambient temperature by using a high-frequency wiring board composed of an insulator having a lower coefficient of thermal conductivity than alumina has and high-frequency lines in at least part of the connecting circuit between the element and an electric signal terminal.

In order to obtain heat insulation between a package 7 and an optoelectronic element 1, mullite is used for forming a microstrip high-frequency wiring board 2A. Mullite has a high frequency loss as alumina does, but has a very low coefficient of thermal conductivity as compared with alumina. When the width of the substrate 2A, length of a heat insulating section, and the thickness of electrode gold are respectively 1mm, 1mm, and 1.5μm and the temperature difference between the package 7 and element 1 is 50°C, the heat conducting quantities of a mullite board and an alumina board respectively become 43mW and 330mW. Therefore, the Peltier current which is required for obtaining a temperature difference of about 50°C can be reduced by 10% when the mullite board is used and the temperature stability of the element 1 against the fluctuation of the ambient temperature can be improved.