An actuator is provided whose expansion in the radial direction can be efficiently translated into longitudinal movement when its length is contracted and extended by injecting a fluid into the tubular body. The fluid injection type actuator includes an actuator body, which is an expansion and contraction section of the actuator. The actuator body is constructed of a cylindrical rubber tube and annular fiber groups inserted and extending longitudinally therein. The annular fiber groups are each a group of fibers, such as glass roving fibers having a diameter of about 10 μm, arranged in an annular array along the circumference of the rubber tube. The arrangement allows the rubber tube to be restrained over the entirety of the actuator body longitudinally when it is expanded radially.