A seal device for a rotary shaft includes a sleeve, a gland, a ring, a guide block, and a fixing member. The guide block has an outer block surface extending circumferentially and abutting against a portion of an inner gland surface of the gland, and an inner block surface extending circumferentially and opposite to the outer block surface. The outer block surface has two circumferentially opposite outer ends. The inner block surface has two circumferentially opposite inner ends. The guide block further has two opposite end faces each of which connects one of the outer ends to one of the inner ends. The inner ends subtend an angle of not larger than 90° at a center line of the gland. At least one of the end faces lies in a plane line that is substantially tangent to an outer peripheral face of the ring.
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