There is provided a vibration suppressing device (50) for a jet pump which is located in a reactor pressure vessel of a boiling water reactor and provided with an inlet mixer pipe (33) coupled to a riser pipe (31) and a diffuser (34) coupled to the inlet mixer pipe (33) by means of a slip joint (40). The vibration suppressing device (50) is provided with an extension sleeve (53) disposed on an upper portion of the diffuser (34) of the jet pump and constitutes an extension flow channel (51) on a downstream side of a forward leak flow in a slip clearance (41) formed between an inner peripheral surface of the diffuser (34) and an outer peripheral surface of the inlet mixer pipe (33), and the extension sleeve (53) has a shape such that the extension flow channel (51) has a region in which a flow channel width (Hex) thereof is constant over a predetermined or longer length.