We have proposed a new application of an arrayed waveguide grating (AWG) for a compact planar spectroscopic sensor to meet both conditions for miniaturization and high wavelength resolution of subnanometer order. In a preliminary experiment, the concentration of the sodium acetate solute was determined with an average deviation of 0.25 wt %. Following this, we designed and fabricated a compact spectroscopic sensor using an AWG with an insertion loss of 4 dB in the visible wavelength range. To improve the sensitivity of the visible AWG spectroscopic sensor, the optimum design of multiple grooves for the liquid sample was carried out using the refractive index and transmittance of chlorophylls a and b as various environmental indicators. We succeeded in discriminating chlorophyll solutions using the AWG sensor. We also obtained a transmittance difference of 1.4 dB, which is two times higher than that in the conventional scheme (0.7 dB) by an absorbency change of 1.0 (optical path: 1.0 mm). Relying on these theoretical and experimental investigations using a visible AWG, we can confirm that a compact AWG-based spectroscopic sensor is effective in bio-medical fields.