There is provided a spindle device utilizing an externally pressurized gas bearing assembly, in which the center of a pulse disc of a rotary encoder can be adjusted to minimize an unbalance of the weight of the pulse disc and such unbalance can be adjusted easily to minimize fluctuation of the pulse disc during rotation and to achieve a highly precise detection of the rotational position consistent with the resolution of the rotary encoder. The spindle device includes an externally pressurized gas bearing assembly (3) for rotatably supporting a shaft (1), a motor (10) for driving the shaft (1), and a rotary encoder (19) for detecting a rotational position. A pulse disc (16) of the rotary encoder (19) is fixed on a mounting surface (S), arranged in the vicinity of one end of the shaft (1) and lying perpendicular to a central axis of the shaft (1), by a fastening member (17) such as a bolt removably inserted through a fastening member inserting portion (7) provided in the pulse disc (16). The mounting surface (S) is an end face of a mounting base (12) fixed to that end of the shaft (1). This mounting base (12) is fixed to the shaft (1) by means of a plurality of radially extending set screws (13).