Rock stress was measured in situ by an improved Downward Compact Conical-ended Borehole Overcoring (DCCBO) technique, which is a type of overcoring method, at Kamaishi mine to further promote the practical use of this technique. Prior to in situ stress measurement, a measuring apparatus (special conical bit, strain cell installation tool and data-logger with a built-in tilt/azimuth sensor) for an HQ-size borehole was developed by improving the apparatus used for a PQ-size borehole. A measurement method based on the determination of strain coefficients depending on the estimated elastic properties of an adhesive under the temperature measured in situ was then proposed. Through the use of in situ tests, we verified that this improved measuring apparatus for a HQ-size borehole works well in a water-filled vertical borehole. Moreover, the proposed measurement method could estimate in situ stresses relatively consistent with those obtained by other measurement methods near this test site. The results show that in situ stresses measured by the proposed measurement method are sufficiently reliable.