To measure the Raman scattering spectrum of a monolayer graphene sample, we describe the design and development of a high-efficiency optical measurement device for operation within the small bore of a high-power magnet at low temperature. For the high-efficiency measurement of light emitted from this small region, we designed a compact confocal optics with lens focusing and tilting systems, and used a piezodriven translation stage that allows micron-scale focus control of the sample position. In addition, we show a detailed technique of adjustment of optical axes, and explain the importance of the focusing and tilting systems. The operation of the device was confirmed by Raman scattering measurements of monolayer graphene on quartz glass with a high signal-to-noise ratio. Though the light collection efficiency of the device is about 240-fold smaller than that of a conventional optical microscope, the device has a sufficient collection efficiency for detecting the Raman scattering light of monolayer graphene.