A catheter for the creation of an atrial septal defect without thoracotomy. The catheter comprises a main flexible tubular housing and a branch tubular housing. A flexible guide wire extends through the main housing and is secured at the distal end thereof and extends outwardly of the distal end. The guide wire can be deformed into a dome shape protruding from a slit opening in the distal end of the main housing by advancing the guide wire from the proximal end of the main housing. A tiny surgical blade disposed in the slit opening of the main housing is elevated to a desired angle by advancing a flexible control wire pivoted to the blade and extending through the main housing. The branch tubular housing communicates with the main housing and is adapted for connecting a syringe or pressure gauge. A pair of rubber stoppers are provided on the main housing.