A body fluid collecting device is used by mounting a tip with a puncture needle and a front end opening at the front end portion thereof for allowing the puncture needle to pass therethrough. The device additionally includes a device body, having a contact part for fitting skin thereto, a tip mounting part having a storage space formed therein capable of storing the tip, a pump for depressurizing the inside of the storage space, and a movable means. The movable means is capable of moving the front end portion of the tip mounting part along a longitudinal direction of the puncture needle, and within a specified range, when the storage space is depressurized by the pump under a condition in which the tip is stored in the storage space, the skin is fitted to the contact part, and the front end opening is sealed.