In this tutorial review, chemical sensors which utilize optical detecting methods are described. Optical probes such as dyes, fluorescent dyes, chemical luminescences are widely-used typical chemical detection tools, and are used also for detection mechanism of chemical sensor device. Reflection, refraction, interference and their spectrometries are common techniques of chemical detection for surface or layers. Plasmonics is unique properties of metal nanoparticles and various sensing devices are developed. Image sensing for chemical distribution or odor space are useful application of optical chemical sensing technique. Fiber-optics array and nano beads array are also described. Finally, terahertz imaging is introduced as a future remote/contactless sensing technology.