We propose an interstitial coaxial antenna that can insert without a catheter for microwave ablation therapy, operating at 2.45 GHz. The cap of the antenna is an arrowhead that bonds copper plate and ceramic plate to suppress heat conduction in the tip direction. The temperature distribution in the improved liver phantom is measured by using the infrared thermography device. As a result, it is clarified that we nearly get the spherical distribution of an assumed tumor in shape.