A disk brake comprising: a caliper in which a piston is caused to slidingly move in a bore of a cylinder under application of a hydraulic pressure and thereby press a brake pad against a disk, wherein: the cylinder comprises cylinders (20, 21) disposed in an opposing relation to each other with the disk sandwiched therebetween; one of the cylinders comprises a pair of vehicle mounting portions extending in a radial direction of the disk for radially mounting the caliper onto a non-rotational portion of a vehicle; characterised in that the other cylinder disposed opposite of the disk from the one cylinder comprises a covering member, which is made of aluminum and is provided at a bottom of said other cylinder, and a cylinder body, which is made of aluminum and has an opening that has an inner diameter smaller than an inner diameter of an internal diameter portion of the bore and is configured to be closed by the covering member; an outer circumference of the covering member and an inner circumference of the opening of the cylinder body are friction stir welded to each other along an entire periphery of the covering member, thereby forming a connecting portion surrounding the covering member in the form of a closed loop; and said other cylinder having the connecting portion and said one cylinder having the vehicle mounting portions are disposed in an opposing relation to each other with the disk sandwiched therebetween. a caliper in which a piston is caused to slidingly move in a bore of a cylinder under application of a hydraulic pressure and thereby press a brake pad against a disk, wherein: the cylinder comprises cylinders (20, 21) disposed in an opposing relation to each other with the disk sandwiched therebetween; one of the cylinders comprises a pair of vehicle mounting portions extending in a radial direction of the disk for radially mounting the caliper onto a non-rotational portion of a vehicle; characterised in that the other cylinder disposed opposite of the disk from the one cylinder comprises a covering member, which is made of aluminum and is provided at a bottom of said other cylinder, and a cylinder body, which is made of aluminum and has an opening that has an inner diameter smaller than an inner diameter of an internal diameter portion of the bore and is configured to be closed by the covering member; an outer circumference of the covering member and an inner circumference of the opening of the cylinder body are friction stir welded to each other along an entire periphery of the covering member, thereby forming a connecting portion surrounding the covering member in the form of a closed loop; and said other cylinder having the connecting portion and said one cylinder having the vehicle mounting portions are disposed in an opposing relation to each other with the disk sandwiched therebetween. the cylinder comprises cylinders (20, 21) disposed in an opposing relation to each other with the disk sandwiched therebetween; one of the cylinders comprises a pair of vehicle mounting portions extending in a radial direction of the disk for radially mounting the caliper onto a non-rotational portion of a vehicle; characterised in that the other cylinder disposed opposite of the disk from the one cylinder comprises a covering member, which is made of aluminum and is provided at a bottom of said other cylinder, and a cylinder body, which is made of aluminum and has an opening that has an inner diameter smaller than an inner diameter of an internal diameter portion of the bore and is configured to be closed by the covering member; an outer circumference of the covering member and an inner circumference of the opening of the cylinder body are friction stir welded to each other along an entire periphery of the covering member, thereby forming a connecting portion surrounding the covering member in the form of a closed loop; and said other cylinder having the connecting portion and said one cylinder having the vehicle mounting portions are disposed in an opposing relation to each other with the disk sandwiched therebetween.