Provided are a decompression mechanism capable of performing a desired pressure reduction by simple operation and having improved operability, a puncture device, a blood analysis device, and a sensor mounting mechanism. A needle puncturing device (100) is provided with: a piston (121) having at one end thereof an end part (121a) for forming a part of a sensor mounting mechanism (130) and at the other end an end part (121b) for slidably supporting a rod (112) of a lancet section (111) a cylinder (122) for slidably containing therein the end section (121b) of the piston (121) and packing (125) mounted to the inner periphery of the end part (121b) of the piston (121) and maintaining the air-tightness of the outer periphery of the rod (112). When the piston (121) is moved in the direction toward the cylinder (122) with a skin contact part (131) in contact with the skin, the volumes of an internal space (140) and a pressure reduction chamber (150) which are sealed by packing (123, 124) are increased to produce a reduced pressure.
