An attachment which is attachable to a game controller including a controller-side slide portion that includes a controller-side first operation button and a controller-side second operation button, the attachment comprising: a strap; an attachment-side slide portion on a first surface of the attachment, the attachment-side slide portion having a first side of a center of the attachment-side slide portion and a second side of the center of the attachment-slide portion opposite to each other in a predetermined slide direction, wherein the attachment-side slide portion is configured to slidably engage with the controller-side slide portion in the slide direction, and wherein the controller-side slide portion is insertable into the attachment-side slide portion from the first side; a stopper portion on the second side of the center of the attachment-side slide portion and configured to limit a slide movement, in an insertion direction, of the controller-side slide portion which has been inserted from the first side of the attachment-side slide portion; an lock portion (49, 631a, 631b) configured to resist the slide movement, in a removal direction opposite to the insertion direction, while the slide movement of the controller-side slide portion is substantially limited by the stopper portion; an attachment-side first operation button on a second surface on a reverse side from the first surface; and an attachment-side second operation button on the second surface, wherein: the attachment-side first operation button includes a first actuation portion configured to move from a first position to a second position, thereby pressing the controller-side first operation button, in response to an operation of pressing the attachment-side first operation button; and the attachment-side second operation button includes a second actuation portion configured to move from a third position to a fourth position, thereby pressing the controller-side second operation button, in response to an operation of pressing the attachment-side second operation button. a strap; an attachment-side slide portion on a first surface of the attachment, the attachment-side slide portion having a first side of a center of the attachment-side slide portion and a second side of the center of the attachment-slide portion opposite to each other in a predetermined slide direction, wherein the attachment-side slide portion is configured to slidably engage with the controller-side slide portion in the slide direction, and wherein the controller-side slide portion is insertable into the attachment-side slide portion from the first side; a stopper portion on the second side of the center of the attachment-side slide portion and configured to limit a slide movement, in an insertion direction, of the controller-side slide portion which has been inserted from the first side of the attachment-side slide portion; an lock portion (49, 631a, 631b) configured to resist the slide movement, in a removal direction opposite to the insertion direction, while the slide movement of the controller-side slide portion is substantially limited by the stopper portion; an attachment-side first operation button on a second surface on a reverse side from the first surface; and an attachment-side second operation button on the second surface, wherein: the attachment-side first operation button includes a first actuation portion configured to move from a first position to a second position, thereby pressing the controller-side first operation button, in response to an operation of pressing the attachment-side first operation button; and the attachment-side second operation button includes a second actuation portion configured to move from a third position to a fourth position, thereby pressing the controller-side second operation button, in response to an operation of pressing the attachment-side second operation button. the attachment-side first operation button includes a first actuation portion configured to move from a first position to a second position, thereby pressing the controller-side first operation button, in response to an operation of pressing the attachment-side first operation button; and the attachment-side second operation button includes a second actuation portion configured to move from a third position to a fourth position, thereby pressing the controller-side second operation button, in response to an operation of pressing the attachment-side second operation button.