An attachment structure for a water heater configured to fix the water heater to an attachment surface such as a wall surface with a metal fitting (1, 2), the attachment structure comprising: a first metal fitting configured to be attached to the attachment surface; and a second metal fitting configured to be attached to a back surface of the water heater, wherein the first metal fitting comprises a wall surface attachment portion configured to be attached to the attachment surface, and a support portion extended from the wall surface attachment portion in a direction separate from the attachment surface, the support portion configured to engage with the second metal fitting and to hang and support a weight of the the second metal fitting comprises an instrument attachment portion configured to be attached to the back surface of the water heater, characterized in that the second metal fitting further comprises an engagement portion (20, 20b) extended from a lower end side of the instrument attachment portion in a direction separate from the water heater, the engagement portion (20, 20b) configured to engage with the support portion of the first metal fitting from above, and a lateral length of the support portion of the first metal fitting is set to enough length to slide the engagement portion (20, 20b) of the second metal fitting in a horizontal direction, the second metal fitting further comprises an extension portion (21, 22) above the engagement portion (20, 20b) at an upper end side of the instrument attachment portion, the extension portion (21, 22) being extended from the instrument attachment portion in a direction separate from the water heater, the extension portion (21, 22) comprises a first extension piece extended obliquely upward in the direction separate from the water heater, and a second extension piece rising from the first extension piece above the water heater in parallel to the attachment surface, the second extension piece is provided with a hole portion (22a, 22b) for wall surface fixing, and when the support portion of the first metal fitting and the engagement portion (20, 20b) of the second metal fitting are in an engagement state, a surface of the second extension piece on an attachment surface side, and a surface of the wall surface attachment portion of the first metal fitting on an attachment surface side are set to be located on a same plane. a first metal fitting configured to be attached to the attachment surface; and a second metal fitting configured to be attached to a back surface of the water heater, wherein the first metal fitting comprises a wall surface attachment portion configured to be attached to the attachment surface, and a support portion extended from the wall surface attachment portion in a direction separate from the attachment surface, the support portion configured to engage with the second metal fitting and to hang and support a weight of the the second metal fitting comprises an instrument attachment portion configured to be attached to the back surface of the water heater, characterized in that the second metal fitting further comprises an engagement portion (20, 20b) extended from a lower end side of the instrument attachment portion in a direction separate from the water heater, the engagement portion (20, 20b) configured to engage with the support portion of the first metal fitting from above, and a lateral length of the support portion of the first metal fitting is set to enough length to slide the engagement portion (20, 20b) of the second metal fitting in a horizontal direction, the second metal fitting further comprises an extension portion (21, 22) above the engagement portion (20, 20b) at an upper end side of the instrument attachment portion, the extension portion (21, 22) being extended from the instrument attachment portion in a direction separate from the water heater, the extension portion (21, 22) comprises a first extension piece extended obliquely upward in the direction separate from the water heater, and a second extension piece rising from the first extension piece above the water heater in parallel to the attachment surface, the second extension piece is provided with a hole portion (22a, 22b) for wall surface fixing, and when the support portion of the first metal fitting and the engagement portion (20, 20b) of the second metal fitting are in an engagement state, a surface of the second extension piece on an attachment surface side, and a surface of the wall surface attachment portion of the first metal fitting on an attachment surface side are set to be located on a same plane.