A cover opener for opening a cover rotationally removed from and fixed to a reagent container, the opener comprising: a shaft provided along a central axis, a rotational drive unit slidable along the central axis relative to the shaft and driven by rotational driving means to rotate about a rotational axis of the cover; a carrier of cylindrical shape, provided at an opposed position with respect to the cover, slidable along the central axis relative to the shaft and including a plurality of protrusions formed for engagement with a plurality of grooves spaced from each other on an upper outer circumference of the cover, the carrier being driven to rotate as the rotational drive unit rotates, as well as to move in the direction of the rotational axis with respect to the rotational drive unit; a cover retainer provided at a lower end of the shaft and being arranged so as to move in a direction of the rotational axis with respect to the rotational drive unit, the cover retainer being configured to become engaged with a concave portion provided on an upper portion of the cover and thereby retain the cover; means, provided between the rotational drive unit and the cover retainer, for biasing the cover retainer in the direction of its rotational axis, towards the cover, as well as for biasing the rotational drive unit in a direction opposite to that direction, the biasing means being a first means for biasing the rotational drive unit and the cover retainer in relative fashion to move away from each other; and means, provided between the rotational drive unit and the carrier, for biasing the carrier in the direction of its rotational axis, towards the cover, as well as for biasing the rotational drive unit in a direction opposite to that direction, the biasing means being a second means for biasing the rotational drive unit and the carrier in relative fashion to move away from each other, characterised in that the cover retainer provided at the lower end of the shaft is configured to act as a stopper against downwards movement of the carrier, and a stopper is provided for abutment of a convex portion of the carrier, thereby limiting upwards movement of the carrier, another stopper is provided at an upper end of the shaft, and a third biasing means is provided at an upper portion of the rotational drive unit for alleviating a shock that may occur between said other stopper and the rotational drive unit. a shaft provided along a central axis, a rotational drive unit slidable along the central axis relative to the shaft and driven by rotational driving means to rotate about a rotational axis of the cover; a carrier of cylindrical shape, provided at an opposed position with respect to the cover, slidable along the central axis relative to the shaft and including a plurality of protrusions formed for engagement with a plurality of grooves spaced from each other on an upper outer circumference of the cover, the carrier being driven to rotate as the rotational drive unit rotates, as well as to move in the direction of the rotational axis with respect to the rotational drive unit; a cover retainer provided at a lower end of the shaft and being arranged so as to move in a direction of the rotational axis with respect to the rotational drive unit, the cover retainer being configured to become engaged with a concave portion provided on an upper portion of the cover and thereby retain the cover; means, provided between the rotational drive unit and the cover retainer, for biasing the cover retainer in the direction of its rotational axis, towards the cover, as well as for biasing the rotational drive unit in a direction opposite to that direction, the biasing means being a first means for biasing the rotational drive unit and the cover retainer in relative fashion to move away from each other; and means, provided between the rotational drive unit and the carrier, for biasing the carrier in the direction of its rotational axis, towards the cover, as well as for biasing the rotational drive unit in a direction opposite to that direction, the biasing means being a second means for biasing the rotational drive unit and the carrier in relative fashion to move away from each other, the cover retainer provided at the lower end of the shaft is configured to act as a stopper against downwards movement of the carrier, and a stopper is provided for abutment of a convex portion of the carrier, thereby limiting upwards movement of the carrier, another stopper is provided at an upper end of the shaft, and a third biasing means is provided at an upper portion of the rotational drive unit for alleviating a shock that may occur between said other stopper and the rotational drive unit.