Knee airbag with tether arrangement

In a knee protecting airbag device (26) of the present invention, an airbag (26) is folded and housed on the front side of the knees (K) of a seated passenger. The airbag (26) is protruded rearward of a vehicle from a protruding opening (17) and expanded and inflated upward, when an inflating gas flows in. And, the airbag (26) is arranged on the front side of the knees (K) of the passenger between a member on the body side (27) and the knees (K) of the passenger thereby to protect the knees (K) of the passenger. The protruding opening (17) has a transverse width size set within a range of 1/8 to 2/3 of the transverse width size of the airbag (26) having completed its expansion and inflation.