A diagnostic imaging catheter includes a drive shaft provided with a signal transmitting and receiving unit at a distal portion thereof and which is rotatable and movable forward and backward, a sheath into which the drive shaft is inserted, a relay connector coupled to a proximal end of the sheath, a support tube provided at an outer circumference of the drive shaft along an axial direction and which moves forward and backward in conjunction with the drive shaft, a seal member provided inside the relay connector and which seals a space between the relay connector and the support tube, and an injection opening located in a region closer to a distal side than the seal member and closer to a proximal side than a distal end of the support tube in a backward movement limit position thereof and via which a liquid is able to be injected into the sheath.