To provide a closing support device for a sliding door, which can prevent the generation of a collision noise and the pinching of a person's finger by preventing a door panel from colliding with a door frame, even if the door panel is operated to be roughly closed.

A self-closing mechanism and an air damper 9 are provided. The self-closing mechanism comprises a reciprocating interlocking piece 41, an assist spring 42 for urging the interlocking piece 41 to be closed, and a switching body 43 for switching the interlocking piece 41 between a connection standby attitude and a connection attitude. The interlocking piece 41 comprises a connection groove 49 with/from which an operation pin 61 of the switching piece 43 can be engaged/disengaged, and a locking piece 47 which can be engaged with/disengaged from a latching part 57 of an inner frame 40. The latching piece 47 is locked to the latching part 57 in a prescribed position, so that the interlocking piece 41 can be positionally held in the connection standby attitude. In the closing of the door panel 1, when the operation pin 61 is engaged with the connection groove 49 so that the interlocking piece 41 can be switched to the connecting attitude, the latching piece 47 is disengaged from the latching part 57. This enables the door panel 1 to be forcedly operated for closing, and concurrently enables a closing operation to be damped by the air damper 9.