To provide a seat belt tightening force-relaxing device having a simple structure to be easily handled and capable of reducing an oppressive feeling given to an occupant by the seat belt.

The seat belt tightening force-relaxing device has: a plate like body 2 arranged between the occupant P and a webbing W when the seat belt is fitted; and an envelope 3 for wrapping the plate like body 2 and the webbing W, wherein the plate like body 2 is arranged in both ends and has a first holding part 21 held by a shoulder anchor 12 and a second holding part 22 held by a tongue 15, and is configured to relax the tightening force T loaded on the occupant P due to the tension of the webbing W by allowing a part of the seat belt device to hold the first holding part 21 and the second holding part 22 to use the counter force F of the plate like body 2.