This paper presents an in-process status monitoring method for ultraprecision machining system and an adaptive control system equipped with the function. The crux of this method is an effective in-process micro sensor which can detect physical behavior around the cutting point during ultraprecision machining process. In this study, we develop a micro-miniaturized platinum resistance thermometry-type sensor with micro-fabrication process and then mount it on the rake surface of a single diamond tool tip nearby the cutting point. Through a series of cutting experiments with an ultraprecision diamond turning machine, we evaluate the developed sensor. As a result, experimental results confirm that the developed sensor has useful characteristics such as high sensitivity, compactness and quick response. In addition, we realize an adaptive control system for ulatraprecision machining process. The developed system maintains a constant temperature around the cutting point by controlling the cutting speed. Adaptive control experiment results confirm the developed system has high potential for detecting small thermal and small dynamic behavior of the cutting point during ultraprecision machining process.