An actuator for use with a fluid channel structure comprises: a stator assembly including stator yokes, and a coil; and a rotor assembly including a housing shaped in a circular hollow-cylinder, an output shaft inserted through the housing, a sleeve, a magnet fixedly attached to the sleeve, and a rotor case. In the actuator described above, the housing includes first and second sealing units having elasticity and disposed at the outer circumference of the circular hollow-cylinder. The first sealing unit makes elastic contact with a wall of a fluid channel of the fluid channel structure, and the rotor case has a cup-like configuration and has an open end portion thereof engaging with the circular hollow-cylinder portion of the housing and pressing on the elastic second sealing unit. Thus, hermetic structure is surely achieved.