We developed an in-situ observation system of the position and temperature at the crystal–solution interface grown under high temperature. A zone furnace of vertical type has an optical window made of quartz glass, and a sample in the furnace can be directly observed from outside through this window. Beam splitter made of an Si single crystal reflects the visible light comming from the sample, and the visible light image is observed using a CCD camera. Infrared light emitted from the sample penetrates the beam splitter, and the infrared image is monitored by an infrared CCD camera (thermoviewer). A growth process of a SiGe mixed crystal was chosen as a model system. Using the newly developed in-situ monitoring system, the position of the growth interface could be monitored with a spatial resolution of 300μm. Temperature distribution around the growth interface could be determined with a spatial resolution of 230μm and an accuracy of ±8°C.