A seedling planting device has a sliding guiding part (30) and a supporting arm (31), the sliding guiding part (30) is arranged on a reverse side of a main seedling carrying table (15A), one end side of the supporting arm (31) is connected with an end part seedling carrying table (15B), the other end is relatively movably guided and supported on the sliding guiding part (30) in the lateral direction, and the supporting arm (31) is such configured that one end side connected with the end part seedling carrying table (15B) can swing around a swinging axis (P1) that the other end side has into a state along an extension direction of the sliding guiding part (30) and a state of erecting towards a surface side of the main seedling carrying table (15A), and the supporting arm (31) is relatively movably supported on the sliding guiding part (30) through a roller (35) for rolling. By adoption of the seedling planting device, sliding operation of the supporting arm relative to the sliding guiding part of a connection supporting device can be performed easily and conveniently with good operability.