In this paper, we propose a small, low cost force sensor based on a commercially available photointerrupter for use in a tendon-driven robotic hand. The proposed sensor consists of a photointerrupter and an elastic frame and is designed to measure tendon tension. Using the design methodology presented in this paper, the force sensor can be made small enough (4 mm × 6 mm × 10 mm) to be inserted into a robotic fingertip and made able to measure a large allowable force (approximately 200 N) with low non-linearity (1.85%) and low hysteresis (1.19%). Due to the characteristics of the photointerrupter, it is possible to overcome the disadvantages associated with traditional miniature force sensors and to have very high resolution (0.1 N) over the entire measurement range (0–200 N). To verify feasibility, we performed force feed-back control using a prototype of the sensor. Experimental results show that the proposed sensor has comparable performance to commercial force sensors of similar size and can be manufactured more simply and economically.