## 1 Introduction  
Recently, the demand for accuracy measurement of microstructures has been remarkable. For example, in cases where micro-holes associated with the nozzles and semiconductor devices have diameters ranging from several micrometers to several ten micrometers, with depths ranging from several tens of micrometers to several hundred micrometers, a measurement probe of a diameter of less than 10 \(\upmu\)m with a high-aspect-ratio is required.  
Numerous papers have been issued on microstructure measurement systems that have many different kinds of probes, such as mechanical probes (e.g., elastic probes [1] and vibration applied probe [2; 3]), optical probes (e.g., fiber probes [4; 5], optical trapping probe [6]) and others (e.g., the atomic force microscope probe [7]). In regard to mechanical probes, the internal shape of the microstructure (e.g., micro hole) can be measured in high reproducibility. However, the probe needs to have high rigidity in order to transmit the force generated at the probe tip to the force detective sensor. Therefore, it is very difficult to develop the probe shaft with a diameter of less than few \(\upmu\)m. The optical probes have the same problems because of the resolution limit of lenses and CCDs. The atomic force microscope has a high measurement resolution, but it cannot be applied to measure the internal shape of microstructures with high aspect ratios. For these reasons, measuring the accuracy of micro holes and grooves with diameters and widths smaller than a few \(\upmu\)m in diameter is extremely difficult because of the difficulty of stylus manufacture and sensing methods with low measurement forces.  
We have been developing a measurement system for microstructures using a stylus made of an optical fiber and prisms [8]. The probe shaft and tip are manufactured using a wet etching device and CO\({}_{2}\) laser processing [9]. In our previous paper [8], the measurement system was improved by using the prisms to expand the measurement region and depth, after which we examined the repeatability of measurement in the X-,Y- and Z-directions, using a pin gauge with a diameter of 100 \(\upmu\)m in the X- and Y-directions and using a step height standard with a height of 189 nm in the Z-direction. In addition, the paper has the result of the long-term drift for 180 min and the expanded uncertainty. In this measurement system, the oblique irradiation of the laser beam was employed to detect the displacement of the stylus shaft generated by contact between the stylus tip and the measured surface, and to expand the measurable area and depth. The incident angle was set to 26.5\({}^{\circ}\). After these evaluations, we discovered that the sensitivity was enhanced with the increase in the incident angle of the laser beam on the stylus shaft. Therefore, in this study, we focus on the effect of the incident angle of light on the measurement sensitivity. First, the enhancement sensitivity is confirmed by using the basic experimental apparatus. Next, the relationship between the incident angle of the laser beam and the sensitivity is theoretically examined using the 3D ray-tracing method. Finally, the factor of the sensitivity enhancement is analyzed using same method.