To provide a catheter with a hardness sensor mechanism capable of lowly invasively and quantitatively measuring the hardness of a lesion inside a tube or inside a celom.

For a catheter tube 2, a piston 3 to be in contact with a body to be detected such as the lesion is provided freely movably back and forth in the longitudinal direction of the catheter tube 2 and silicone gel 13 for transmitting a pressure based on the displacement of the piston 3 is provided. For the silicone gel 13, a vibration body 11 for imparting prescribed vibration is provided and a pressure sensor 5 for detecting the pressure impressed to the silicone gel 13 is provided. When the catheter tube 2 is put into the tube or the like and the piston 3 is brought into contact with the lesion or the like which is the body to be detected, the movement of the piston 3 is limited corresponding to the hardness of the body to be detected such as the lesion with which the piston 3 is brought into contact. By the limitation of the displacement of the piston 3, the pressure based on the vibration impressed from the vibration body 11 to the silicone gel 13 is increased and the pressure sensor 5 detects the increased pressure.