An information processing system including an information processing device having a housing with a front surface, side surfaces and a back surface, the information processing device further comprising a camera provided on one of the side surfaces of the housing of the information processing device and a screen provided on the front surface of the housing, and an accessory which can be attached to the information processing device, wherein: the accessory comprising: a housing of which an inside is visible from an outside thereof through at least a portion thereof; a movable operation section (22A-22D; 41) at least a portion of which is exposed on the outside of the housing of the accessory; and a movable portion (23A-23D; 43) placed at a position inside the housing of the accessory that is visible from the outside of the housing of the accessory through the portion of the housing of the accessory, wherein a position, or an attitude, or both position and attitude of the movable portion changes in response to the operation section being operated; the accessory is attached to the information processing device at such a position that the camera can capture an image of the inside of the housing of the accessory through the portion of the housing of the accessory; and the information processing device comprises: an operation determination unit (14, S13) for determining an operation performed on the operation section based on a camera image captured by the camera and including an image of the movable portion; and a process performing unit (14, S14) for performing a predetermined process based on the operation determined by the operation determination unit. the accessory comprising: a housing of which an inside is visible from an outside thereof through at least a portion thereof; a movable operation section (22A-22D; 41) at least a portion of which is exposed on the outside of the housing of the accessory; and a movable portion (23A-23D; 43) placed at a position inside the housing of the accessory that is visible from the outside of the housing of the accessory through the portion of the housing of the accessory, wherein a position, or an attitude, or both position and attitude of the movable portion changes in response to the operation section being operated; a housing of which an inside is visible from an outside thereof through at least a portion thereof; a movable operation section (22A-22D; 41) at least a portion of which is exposed on the outside of the housing of the accessory; and a movable portion (23A-23D; 43) placed at a position inside the housing of the accessory that is visible from the outside of the housing of the accessory through the portion of the housing of the accessory, wherein a position, or an attitude, or both position and attitude of the movable portion changes in response to the operation section being operated; the accessory is attached to the information processing device at such a position that the camera can capture an image of the inside of the housing of the accessory through the portion of the housing of the accessory; and the information processing device comprises: an operation determination unit (14, S13) for determining an operation performed on the operation section based on a camera image captured by the camera and including an image of the movable portion; and a process performing unit (14, S14) for performing a predetermined process based on the operation determined by the operation determination unit. an operation determination unit (14, S13) for determining an operation performed on the operation section based on a camera image captured by the camera and including an image of the movable portion; and a process performing unit (14, S14) for performing a predetermined process based on the operation determined by the operation determination unit.