To provide a forced deterioration device and a forced deterioration method for a seatbelt capable of obtaining samples having deterioration tendency similar to those actually deteriorated with the passage of time in a short time and with high reproducibility.

The forced deterioration device 20 of the seatbelt device has a holding means 27 for holding a retractor 12, a through-anchor 14, and a webbing 11 in almost the same position relation as being boarded on a vehicle; a webbing extraction winding means 23 for extracting the webbing in almost the same state as a non-used state to almost the same state as an occupant restraint state from the retractor 12 and winding the webbing 11 round the retractor 12 to almost the same state as the non-used state; a dust 35 giving means 30 for giving the dust to the specified position of the webbing 11 every the specified frequencies of extraction of the webbing 11; and a soiled solution giving means 40 for giving a soiled solution 43 to the specified position of the webbing 11 every the specified frequencies of the extraction of the webbing 11.