Nowadays, minimum invasive surgery using intravascular surgical tools is getting greater attention in the medical field. This technique allows us to reduce physical pain and sequelae from patients. For example, a catheter is one of the medical tools which is used in endovascular surgeries. The catheter is frequently used for neurosurgical operations. We propose a new type of force sensor for intravascular neurosurgery. This micro force sensor can be installed in the tip of the catheter. We developed a prototype micro force sensor whose diameter is 1.6 mm and length is 12 mm. Using this micro force sensor, we can measure the contact force between the catheter and blood vessels. This is quite important to reduce the unnecessary contact force between the catheter and blood vessels. In this paper, we show the structure of the micro force sensor and its characteristics, and we also show the results of in vivo experiment.