To improve high-frequency transmission characteristics, maintain cost at a low price and enable high-speed operation of at least 10 Gb/s, by reducing electrical reflections due to low impedance, at penetrating parts of high-frequency pins in a stem.

This optical semiconductor integrated device is provided with an optical semiconductor element module and a substrate 300 for an integrated circuit; an LD 40, the stem 10 for retaining dielectric 77 wherein a pair of the high-frequency signal pins 41a, 41b constituting a differential line penetrate and are fixed, and microstrip differential line substrates 46, 47 having a pair of differential lines. The substrate 300 is provided with an integrated circuit 100, wherein a differential signal is transmitted to the LD 40, and a grounded coplanar differential line 70, having a pair of differential lines which connect the integrated circuit 100 and a pair of the high-frequency signal pins 41a, 41b. Stubs 302a, 302b, whose characteristic impedance is lower than that of a pair of differential signal lines 71a, 71b are formed, respectively, on the differential signal lines 71a, 71b of the grounded coplanar differential line 70.

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