A blood test apparatus having a simple constitution whereby stable measurement can be conducted by surely sampling the blood in an amount being small but sufficient for the test without placing too much burden on a patient. When a first skin contact sensor of this apparatus detects the skin, driving of a negative pressure unit is initiated. Thus, the skin rises and comes into contact with a second skin contact sensor. After piercing into the skin, the negative pressure supply is once ceased. Next, the negative pressure is applied again for a definite period of time. Thus, the opening in the skin is broadened, which facilitates the flow out of the blood.