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 includes: a piston having at one end thereof an end part for forming a part of a sensor mounting mechanism and at the other end an end part for slidably supporting a rod of a lancet section a cylinder for slidably containing therein the end section of the piston and packing mounted to the inner periphery of the end part of the piston and maintaining the air-tightness of the outer periphery of the rod. When the piston is moved in the direction toward the cylinder with a skin contact part in contact with the skin, the volumes of an internal space and a pressure reduction chamber are increased to produce a reduced pressure.
