In this study, a half-circled cavity based microdisk laser diode is proposed and demonstrated experimentally for an integrated photonic biochemical sensor. Conventional microdisk sensors have limitations in optical coupling and reproducibility. In order to overcome these drawbacks, we design a novel half-circled micro disk laser (HC-MDL) which is easy to manufacture and has optical output directionality. The Q-factor of the fabricated HC-MDL was measured as 7.72 × 106 using the self-heterodyne method and the side mode suppression ratio was measured as 23 dB. Moreover, gas sensing experiments were performed using the HC-MDL sensor. A wavelength shift response of 14.21 pm was obtained for 100 ppb dimethyl methylphosphonate (DMMP) gas and that of 14.70 pm was obtained for 1 ppm ethanol gas. These results indicate the possibility of highly sensitive gas detection at ppb levels using HC-MDL. This attractive feature of the HC-MDL sensor is believed to be very useful for a wide variety of optical biochemical sensor applications.