To provide a medical tool capable of rapidly and securely enlarging a diameter in a deformation part which is provided at a linear body.

A catheter 1A is used by insertion into a living body, and includes a long and flexible wire body 2A; and the deformation part 3A arranged in the middle of the wire body 2A and deformed to be enlarged in diameter. The deformation part 3A includes an actuator 4A having two electrically active polymers 41a to be deformed by energization and three electrodes 42a, 42a, 42b for performing energization in each electrically active polymer 41a, so as to be operated by performing energization between the respective electrodes 42a and 42b.