An ultrasonic diagnostic apparatus includes an insertion section formed by a flexible elongate tube having a distal end portion and a proximal end portion. A driving shaft passes through the flexible elongate tube of the insertion section between a control section at the proximal end of the insertion section, and an ultrasonic transducer arranged for rotation at the distal end portion of the insertion section. The driving shaft includes a flexible elongate member formed by a pair of coils closely fitted coaxially one within the other. The coils have different winding directions thus providing circumferential rigidity when the driving shaft is rotated by a driving source at the control section. A signal cable is routed through the inside space of the coils forming the driving shaft and connects at one end with the transducer. The other end of the signal cable connects with a rotary terminal arrangement at the end of the driving shaft located at the control section.