We experimentally demonstrate a transverse magnetic (TM) mode laser in photonic crystal (PhC) slab structure at room temperature. This study proposes PhC nanobeam (NB) cavity to support high-quality-factor ($Q$-factor) TM mode. For large and complete band gap, the NB structures consist of air holes thick dielectric slab. The is optimized numerically for high-$Q$-factor over 1 000 by reducing ra...