Provided is a medical expansion instrument with which an operation to open a puncture hole and expand the puncture hole can be performed easily, with which a guide wire can be used during operation if necessary, and with which an optimal mode of use that does not injure the body cavity can be suitably selected. A base end portion of a second dilator (40) is secured to the front side of a case body (11), and a distal end portion of the second dilator is formed with a tapered shape. A distal end portion of a first dilator (30) is extended and retracted from the distal end portion of the second dilator (40). A distal end portion of a puncture needle (20) is extended and retracted from the distal end portion of the first dilator (30). A guide wire (A) can be inserted through the puncture needle (20). The distal end portions of the puncture needle (20) and the first dilator (30) can be caused to protrude by means of an operating portion (50), and the respective protruding states are maintained by means of locking portions (60, 70). Further, the distal end portion of the puncture needle (20) can be retracted by means of a second lock releasing portion (75), after which the distal end portion of the first dilator (30) can also be retracted by means of a first lock releasing portion (65).Linvention concerne un instrument médical délargissement avec lequel il est possible de réaliser facilement une opération pour ouvrir un orifice de ponction et élargir lorifice de ponction, avec lequel un fil guide peut être utilisé pendant lopération si nécessaire et avec lequel on peut sélectionner de manière adaptée un mode optimal dutilisation qui ne provoque pas de lésion à la cavité corporelle. Une partie dextrémité de base dun second dilatateur (40) est fixée sur le côté avant dun corps de boîtier (11) et une partie dextrémité distale du second dilatateur a une forme conique. Une partie dextrémité distale dun premier dilatateur (30) est étendue et rétractée à partir de la partie dext
