A wind field over the Lake Biwa is very complex and variable since the wind field is influenced by sea and land breezes from Osaka, Ise and Wakasa Bays, and the Lake Biwa is surrounded by mountains. In this study, we conducted a three-dimensional numerical simulation of the wind field with HOTMAC of a mesoscale meteorological simulation model. And we verified the numerical simulation method by making a comparison between calculated result and some preceding observational data and examined a relationship between the wind field and the extended sea breeze. The simulation result show that a northwest wind which is the most dominant wind system is caused by the sea breeze from the the Wakasa Bay and that the lake breese is strongly influenced by the extended sea breese.