Authors designed and built a mold-type slit rheometer. The slit rheometer has a vertical flow channel and interchangeable cores with various slit thicknesses. It is installed in an injection molding machine like a general mold. The melt is supplied by the plasticizing unit of the injection molding machine. The melt temperature supplied by the plasticizing unit varies by the plasticizing conditions and the shear heating effect in the nozzle and the sprue. In this study, the effects of the plasticizing conditions and the shear heating in the nozzle and the sprue were examined, and the influence of the melt temperature variation on the viscosity measurement was analyzed experimentally and numerically. A temperature sensor was designed to measure the melt temperature in the sprue. Its tip is fully immersed in the melt to have a higher sensitivity. Two resins with different thermal sensitivities were used to examine the effect of resin on the melt temperature variation. To check the shear heating effect in the nozzle and the sprue, two sets of the nozzle and the sprue with different orifice sizes were used. The numerical analysis was done by commercial software (Moldex3D) to check the melt temperature from the nozzle to the slit channel. The viscosity of long-fiber reinforced polypropylene (PP) was measured with various slit channel thicknesses to verify the reproducibility. The standard deviation of the viscosity values against the Cross model fitting curve was 3.01 Pa·s. The viscosity of acrylonitrile butadiene styrene (ABS) measured using the slit rheometer was compared with values obtained from a capillary rheometer and commercial database.