In this paper, we focused on the medical procedure to insert needles under computer tomography (CT) guidance as a target for the robotization of medical surgery, which is one of Interventional Radiology (IR). IR is a general term for treatments that use devices to visualize patients’ bodies. During surgery, our developed robot, known as Zerobot, specializes in inserting a needle into the patient under CT guidance. Its surgery is less invasive and more effective in treating small cancer tumors because the temperature of the inserted needle tip is controlled. Zerobot was originally designed to be remotely controlled by doctors, and first-in-human feasibility trials in 2018 confirmed its surgical capability. We are currently focusing on the automatic insertion function to reduce the workload of doctors. In an animal experiment, Zerobot was unable to insert the needle into the animal during IR surgery if the needle was bent. As a result, the goal of this research is to have the robot function automatically so that the needle does not bend during surgery. We propose a method for estimating the form of the needle using a force sensor. There are three different types of needle forms. Next, the proposed method distinguishes between these needle forms by measuring the difference in force sensor data when the needle root is moved slightly. In addition, we conducted tests to confirm the efficacy of the proposed method.