17mm, so a little bit smaller than Tabanus sudeticus.
Looked like a fly or funny mosquito. Saw it on the ceiling in the kitchen in the morning and the evening it was still around, so took e few pics. Great surprise! It is a little longhorn beetle.
Came to the light in the house?
六星白天牛Olenecamptus taiwanus Dillon et Dillon, 1948
http://gaga.biodiv.tw/new23/000x/s6-632.htm