In the floating zone method with an infrared convergence type heater, the contact between a feed rod and a growing crystal is one of the most serious problems to grow a large-diameter crystal because the contact cause the instability or collapse of the melt zone. In order to examine the possibility to grow large-size rutile single crystals by the floating zone method, the shapes of the solid-liquid interface were investigated as a function of crystal diameter and rotation rate. In the case of growing a 10 mm diameter crystal, the height of interface was easily decreased with increasing crystal rotation rate, which enhanced the strength of forced convection in the melt zone. However, the height of interface of a 13 mm diameter crystal was not significantly reduced despite the increased rotation rate because of the increased melt volume. This result indicates the difficulty in the floating zone growth of large-diameter rutile single crystals only by increasing the rotation rate up to 60 rpm.