A medical device which comprises an indwelling needle assembly comprising an inner needle which has a sharp needle tip at the end thereof and a hollow outer needle into which the inner needle is inserted and a suction means by which blood is sucked through the indwelling needle assembly in an indwelling state where the indwelling needle assembly is pierced into a living body and indwells there, wherein the suction means is provided with a suction power-controlling system which controls the suction power thereof. Between the outer peripheral face of the inner needle and the inner peripheral face of the outer needle of this medical device, a space communicating with the suction means is formed along the longitudinal direction of the indwelling needle assembly and blood flows into this space. In this space, the suction power controlled by the suction power-controlling system is set lower by 5 mmHg or more than the atmospheric pressure.
