A chemical-liquid injector includes a piston-driving mechanism (130) that moves a piston member of a syringe containing a contrast medium, which includes an actuator and a ram member which is moved back and forth by the actuator, a control circuit (150) which is electrically connected to the actuator, and an operating knob unit (170) which includes an operating knob that is to be operated by an operator, and a rotation sensor that outputs an electric signal corresponding to a rotation of the operating knob. The control circuit is configured to generate a predetermined control signal accordingly, on the basis of a signal from the rotation sensor, and the piston-driving mechanism is operated according to the control signal.