Si-selective epitaxial growth (Si-SEG) with Si2H6 gas is performed on clean Si(001)-2x1 open linear windows in an ultrathin SiO2 mask formed by electron-beam-induced selective thermal decomposition (EB-STD) to fabricate a Si nanowire. We demonstrate that Si-SEG is possible in a 15-nm-wide Si(001)-2x1 open linear window formed by EB-STD. However, the width of the Si wire increases with Si growth, because the oxide mask decomposes thermally at the oxide/Si(001)2x1 boundary. An ultrathin oxide layer grown at higher temperature is effective to suppress the widening of the Si wire, even if the oxide thickness is not changed.