A guide device can be inserted into the body in a simple and easy manner, can be used by inserting it from the xiphoid process to selectively and easily perforate the pericardium, and enables reliable detection that it has entered the pericardium. Provided is a guide device (1) including a cylindrical insertion part (2) that is inserted into a body and that opens near a distal end thereof and on a proximal side thereof, a perforating part (3a) that is disposed at the distal end of the insertion part (2) and that perforates tissue, a moving part (3) disposed near the distal end of the insertion part (2) so as to be displaceable between a nearby position near the insertion part (2) and a farther position farther away from the insertion part (2) than the nearby position, a biasing mechanism (4) that biases the moving part (3) in a direction away from the insertion part (2), and a displacement-detecting mechanism (5a) that detects displacement of the moving part (3) in the body.