A probe for insertion into a living body includes a sheath configured to be inserted into a living body, and a shaft positioned in the sheath. The shaft includes a tip portion configured to obtain diagnostic data at a distal portion thereof. The sheath includes a resin layer or a metal layer of at least one layer, and wherein the resin layer or the metal layer includes a spiral-shaped slit which is continuous from a distal portion to a proximal portion. The spiral-shaped slit includes a distal slit portion having the highest slit density, a middle slit portion which is continuous with the distal slit portion and which also has a lower slit density than the distal slit portion, and a proximal slit portion which is continuous with the middle slit portion and which also has a higher slit density than the middle slit portion.
