A centesis instrument, wherein a patient applies a contact part to a palm, a finger, or an arm in the state of gripping an outer tube by hand and presses the outer tube against its measured portion. A third coiled spring is compressed to increase spring power. When the outer tube reaches a prescribed position, the tip face of the vertical portion of a centesis button reaches a hole part, and a projected part is allowed to be pressed. When the patient presses a button body from a direction perpendicular to the axis of the button body by using a finger at that position, a locking piece is separated from a locking part and the centesis needle of a centesis needle unit is instantaneously projected from the opening part of the contact part to the measured portion by the spring power of the third coiled spring for centesis. Thus, a prescribed amount of blood can be taken out from the measured portion.