<p>A simple and quick in situ measurement technique for liquid viscosity is expected to enhance the quality of products and the production efficiency in many fields of engineering such as food production, film coating, medical application, and so on. For example, the in-process viscosity measurement is required to determine the economic feasibility of recovering hydrocarbon from subterranean strata. The real-time viscosity monitoring of organic liquid thin films can realize the control of the rheological and morphological characteristics of film-devices as well as the quality of film. Furthermore, the simple and fast measurement of blood viscosity realizes a diagnosis of cardiovascular disorders with new perspective. Therefore, in situ viscosity measurement could provide a breakthrough in sensing under specific conditions. However, conventional viscometers are inapplicable to in situ measurement because the contact-manner is utilized in these methods, and these methods require a long time for measurement. In the present study, a novel optical hand-held viscosity sensor (OHVS) has been developed to realize in situ viscosity measurement. The proposed sensor enables non-contact and high-speed viscosity measurement based on the optical measurement method called laser-induced capillary wave method. In order to robustly detect the optical signal which contains the information of the viscosity under hand-held condition, we developed the incident angle control system and integrated on the compact handy sensor. Through the experimental study, the irradiation angle was stabilized vertically under hand-held condition by the incident angle control system, and the standard deviation of the irradiation angle during control was smaller than 0.03 degrees that was enough to detect the viscosity signal. Finally, the optical signal which contains the information of viscosity has been observed by OHVS under hand-held condition. The applicability of the proposed sensor for the simple and quick measurement was verified.</p>