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.