To restrain abrasion of a seal material by preventing the shakes of a cap in a simple structure.

This resonator device for a high-pressure fuel pump is mounted among a casing 18 forming a fuel passage 22, a cover 29 mounted on the casing 18, the casing 18 and the cover 29, and forms a fuel passage chamber 32 between it and the fuel passage 22 of the casing 18. A volume chamber 33 is formed between it and the cover 29, a cap 23 for communicating the volume chamber 33 with the fuel passage chamber 32 through an orifice 23b is provided, and the outside diameter B on the fuel passage chamber 32 side in the cap 23 is set to the outside diameter smaller than the outside diameter A on the volume chamber 33 side.