In this paper, we propose a measurement method of 3-axis fingertip force which works when playing a string instrument. A small and thin 3-axis tactile sensor was fabricated to attach to the first finger. The force sensing elements were formed within 1 mm×1 mm on the sensor chip, and the thickness of the sensor was 3 mm. A guitar was used as a string instrument. Using the proposed method, the 3-axis fingertip force was measured in two cases; (a) when the guitar string was held down incorrectly; (b) when the guitar string was held down properly. The data of the 3-axis tactile sensor and the guitar sound were recorded simultaneously. Comparing both cases, it was indicated that the number of the guitar sound's overtone spectra was increased when the string vibration did not propagate through the finger. This proposed method will develop a significant way to analyze the relationship between musicians' fingertip force and instrument sounds.