A linear guide device comprising: a mounting hole on a guide rail, the mounting hole comprising a counterbore part configured to accommodate a head part of a bolt and a shaft accommodation part configured to accommodate a shaft part of the bolt; and a cap of the linear guide device, the cap being configured to close the counterbore part of the mounting hole arranged on the guide rail of the linear guide device, by being hit and press-fitted into the counterbore part with a hitting jig, the cap comprising: a receiving part configured to receive hitting of the hitting jig; a fitting part extending from the receiving part, having an outer diameter to be an interference with the counterbore part, and being press-fitted into the counterbore part; a foreign material receiving groove at an outer circumference of the fitting part, and configured to receive a foreign material generated from the fitting part when the cap is hit and inserted into the counterbore part; a gas discharge groove extending to reach an inside of the counterbore part from the foreign material receiving groove, with the fitting part being press-fitted into the counterbore part at the outer circumference of the fitting part, and configured to discharge a gas in the inside of the counterbore part to a foreign material receiving groove side; and the fitting part comprises a taper part configured to guide introduction of the fitting part into the counterbore part when the cap is hit, the taper part being arranged at the outer circumference and a bottom surface of the fitting part, characterized in that when L represents a depth of the gas discharge groove, φD represents an outer diameter of the fitting part, and φd represents an inner diameter of the taper part, the depth L of the gas discharge groove is set to fall within a range of 0 < L < (φD-φd)/2; and in that two of the gas discharge grooves are arranged at equal intervals at the circumference of the fitting part formed to have a cylindrical shape. a mounting hole on a guide rail, the mounting hole comprising a counterbore part configured to accommodate a head part of a bolt and a shaft accommodation part configured to accommodate a shaft part of the bolt; and a cap of the linear guide device, the cap being configured to close the counterbore part of the mounting hole arranged on the guide rail of the linear guide device, by being hit and press-fitted into the counterbore part with a hitting jig, the cap comprising: a receiving part configured to receive hitting of the hitting jig; a fitting part extending from the receiving part, having an outer diameter to be an interference with the counterbore part, and being press-fitted into the counterbore part; a foreign material receiving groove at an outer circumference of the fitting part, and configured to receive a foreign material generated from the fitting part when the cap is hit and inserted into the counterbore part; a gas discharge groove extending to reach an inside of the counterbore part from the foreign material receiving groove, with the fitting part being press-fitted into the counterbore part at the outer circumference of the fitting part, and configured to discharge a gas in the inside of the counterbore part to a foreign material receiving groove side; and the fitting part comprises a taper part configured to guide introduction of the fitting part into the counterbore part when the cap is hit, the taper part being arranged at the outer circumference and a bottom surface of the fitting part, a receiving part configured to receive hitting of the hitting jig; a fitting part extending from the receiving part, having an outer diameter to be an interference with the counterbore part, and being press-fitted into the counterbore part; a foreign material receiving groove at an outer circumference of the fitting part, and configured to receive a foreign material generated from the fitting part when the cap is hit and inserted into the counterbore part; a gas discharge groove extending to reach an inside of the counterbore part from the foreign material receiving groove, with the fitting part being press-fitted into the counterbore part at the outer circumference of the fitting part, and configured to discharge a gas in the inside of the counterbore part to a foreign material receiving groove side; and the fitting part comprises a taper part configured to guide introduction of the fitting part into the counterbore part when the cap is hit, the taper part being arranged at the outer circumference and a bottom surface of the fitting part, characterized in that when L represents a depth of the gas discharge groove, φD represents an outer diameter of the fitting part, and φd represents an inner diameter of the taper part, the depth L of the gas discharge groove is set to fall within a range of 0 < L < (φD-φd)/2; and in that two of the gas discharge grooves are arranged at equal intervals at the circumference of the fitting part formed to have a cylindrical shape.