The present invention provides an optical modulation element module capable of stably operating for long periods without limiting the operating conditions. An optical modulation element module comprising an optical modulation element 12, a case 11 with said optical modulation element being integrated thereto, input output fibers 13, 14 being connected to said optical modulation element and also guided to an exterior of said case, a terminal for introducing a modulating signal and a bias adjustment voltage that are to be applied to said optical modulation element, said case being hermetically sealed, is characterized in that said case 11 is made of a material having almost the same coefficient of linear expansion as the optical modulation element 12, said optical modulation element is affixed to said case by using a thermoplastic and high-elastic adhesive material 19, a linear expansion difference between each material of said case and said fiber is less than 16×10−6/° C., and a distance between a fixed point of said fiber with said case, and a fixed point of said fiber with said optical modulation element is more than 3 mm.