Disclosed herein is a sensor attached to viscera to measure a change in length, in which the sensor includes a first layer having a first surface attached to the viscera and a second surface opposite to the first surface, a second layer having a third surface attached to the second surface and a fourth surface opposite to the third surface and expanded or contracted as the viscera is expanded or contracted, and a third layer having a fifth surface attached to the fourth surface and a sixth surface opposite to the fifth surface and attached to a catheter, and the first layer to the third layer are a film including silk fibroin and a calcium compound.