To provide an attachment capable of economically converting a single baby car into a double baby car and capable of being compactly folded.

This attachment is used to convert a compactly foldable single baby car 1 into a double baby car. The right and left wheeled attachment leg bars 21, 21 are provided with connecting means 25 to the hand bars 9 of the single baby car 1, angle regulating members 23 of the attachment leg bars 21 to the single baby car 1, and an interval regulating member 22 between the right and left attachment leg bars 21, 21 so that they can be switched to the regulated or released state. A flexible seating seat 24 is provided across the right and left attachment leg bars 21. 21.