  Quantum mechanics predicts superposition of position states even for
macroscopic objects. Recently, the use of a quasi-freely suspended mirror
combined with laser was proposed to prepare such states, by M\"uller-Ebhardt et
al. [Phys.Rev.Lett.100, 013601 (2008)]. One of the key milestones towards this
goal is the preparation of the mechanical oscillator mainly driven by quantum
back-action, which identifies the connection between the object and quantumness
of the light. Here, we describe development of a suspended 5-mg mirror driven
by quantum back-action larger than thermal fluctuating force by a factor of
1.4$\pm$0.2 at 325 Hz,which is confirmed by using a triangular optical cavity.
