An operation apparatus mountable to one hand of a user, the operation apparatus comprising: a casing; an operation section (32, 33, 34, 35) that is provided in the casing and that can be operated by the one hand; and a mounting member by which the casing is mounted to the one hand; wherein the mounting member includes: an attachment member that is attached to the casing, and a band member that is attached to the attachment member and that is configured to be wound along a palm and a back of the one hand; and the band member including: a main body member; a belt-shaped member that is connected to the main body member at one end thereof and configured to be disposed on the back of the one hand when the band member is mounted on the one hand, a sliding mechanism comprising a slide member which slides relative to the main body member to extend or contract an inside circumference size of the band member, and a link member to which the main body member and the other end of the belt-shaped member are connected; and a locking member that locks the sliding mechanism in a state in which the sliding mechanism has slid to contract the inside circumference size, in which the sliding mechanism comprises a first contact surface configured to contact the palm of the one hand when the band member is mounted on the one hand. a casing; an operation section (32, 33, 34, 35) that is provided in the casing and that can be operated by the one hand; and a mounting member by which the casing is mounted to the one hand; wherein the mounting member includes: an attachment member that is attached to the casing, and a band member that is attached to the attachment member and that is configured to be wound along a palm and a back of the one hand; and the band member including: a main body member; a belt-shaped member that is connected to the main body member at one end thereof and configured to be disposed on the back of the one hand when the band member is mounted on the one hand, a sliding mechanism comprising a slide member which slides relative to the main body member to extend or contract an inside circumference size of the band member, and a link member to which the main body member and the other end of the belt-shaped member are connected; and a locking member that locks the sliding mechanism in a state in which the sliding mechanism has slid to contract the inside circumference size, in which the sliding mechanism comprises a first contact surface configured to contact the palm of the one hand when the band member is mounted on the one hand. an attachment member that is attached to the casing, and a band member that is attached to the attachment member and that is configured to be wound along a palm and a back of the one hand; and the band member including: a main body member; a belt-shaped member that is connected to the main body member at one end thereof and configured to be disposed on the back of the one hand when the band member is mounted on the one hand, a sliding mechanism comprising a slide member which slides relative to the main body member to extend or contract an inside circumference size of the band member, and a link member to which the main body member and the other end of the belt-shaped member are connected; and a locking member that locks the sliding mechanism in a state in which the sliding mechanism has slid to contract the inside circumference size, in which the sliding mechanism comprises a first contact surface configured to contact the palm of the one hand when the band member is mounted on the one hand. a main body member; a belt-shaped member that is connected to the main body member at one end thereof and configured to be disposed on the back of the one hand when the band member is mounted on the one hand, a sliding mechanism comprising a slide member which slides relative to the main body member to extend or contract an inside circumference size of the band member, and a link member to which the main body member and the other end of the belt-shaped member are connected; and a locking member that locks the sliding mechanism in a state in which the sliding mechanism has slid to contract the inside circumference size, in which the sliding mechanism comprises a first contact surface configured to contact the palm of the one hand when the band member is mounted on the one hand.