To prevent misdelivery by securely checking whether a DP bag to be sent to a different sending destination is mixed in a DP bag group sorted by destinations of DP bags.

The misdelivery preventing method includes a plurality of steps. Namely, DP bag information (bar code) added to a 1st DP bag in a DP bag group sorted by sending destinations such as collection and delivery courses is read from the DP bag (step S16). Then bar codes of second and succeeding DP bags to be checked in the DP bag are read to decide whether the second and succeeding DP bags belong to the DP bag group sorted by the sending destinations, one after another, according to the bar code read from the first DP bag and the bar codes read from the second and succeeding DP bags (step S18). Then information showing whether the sending destinations of the second and succeeding DP bags are different from the sending destination of the first DP bag is outputted (steps S20, S22).