The present invention is featured in that a plurality of transfer pieces composing a transfer conveyer are connected to be able to be piled vertically spirally, the transfer pieces having pairs of spacer members each of which is formed to have an inside-part and an outside part continuing to the inside-part and offsetting outward in lateral direction perpendicular to the transfer direction, an engaging part having a contact face extending in the direction parallel to the transfer direction being formed at the upper and lower end part of the outside-part of the spacer member; the transfer pieces can be piled such that vertically adjacent spacer members contact to each other with the contact face of the upper and lower engaging part; an inside chain and an outside chain are provided for allowing the inner side and outer side spacer members to ride on respective chain to advance the transfer conveyer; and the inside chain and outside chain are driven by a single motor.