A semiconductor device having a superior connection reliability is obtained by providing a buffer body for absorbing the difference of thermal expansion between the mounting substrate and the semiconductor element in a semiconductor package structure, even if an organic material is used for the mounting substrate. A film material is used as the body for buffering the thermal stress generated by the difference in thermal expansion between the mounting substrate and the semiconductor element. The film material has modulus of elasticity of at least 1 MPa in the reflow temperature range (200 250.degree. C.).