The present invention relates to the provision of a cylinder apparatus that is able to maintain a superior join condition, and has: a cylindrical cylinder (11); an end portion member (12) that is placed on the end portion side of the cylinder (11); and a weld portion (13) that fixes an outer circumferential side of the cylinder (11), when this has been engaged with an inner circumferential side of the end portion member (12), to one end of the end portion member (12) by means of a weld, wherein abutting portions (21), which abut in the circumferential direction against a contact portion (60) of the end portion member (12) and notch portions (20) are provided alternatingly in the circumferential direction on the end portion side of the cylinder (11), and the notch portions (20) form communicating passages (69) that connect the weld portion (13) to an inner circumferential side space (67) inside the cylinder (11) or end portion member (12).