An airbag apparatus comprising: an airbag that is planar before supply of inflation gas and is inflated by supply of the inflation gas; an inflation portion (46; 125) provided in the airbag; a partition member (50; 50F; 50H; 50L; 50R; 50U) that partitions the inflation portion (46; 125) into at least an upstream inflation portion (47; 88; 91; 93; 97; 99; 101; 106; 108; 111; 112; 127) and a downstream inflation portion (48; 89; 92; 94; 98; 100; 102; 107; 113; 129); and a pressure regulating valve that is closed at an early stage of a supply period of the inflation gas to the inflation portion (46; 125), thereby restricting flow of the inflation gas from the upstream inflation portion (47; 88; 91; 93; 97; 99; 101; 106; 108; 111; 112; 127) to the downstream inflation portion (48; 89; 92; 94; 98; 100; 102; 107; 113; 129), and is opened from the middle of the supply period to cancel the restriction, wherein the partition member (50; 50F; 50H; 50L; 50R; 50U) has a length in a longitudinal direction that is longer than a length in a transverse direction perpendicular to the longitudinal direction when being tensioned to be planar upon inflation of the inflation portion (46; 125), and the pressure regulating valve includes: a slit-like inner opening that is provided on the partition member (50; 50F; 50H; 50L; 50R; 50U) and extends in the transverse direction of the partition member (50; 50F; 50H; 50L; 50R; 50U); and a pair of valve body portions (73, 74; 78, 79) that are provided around the inner opening and are selectively close to and separated from each other. an airbag that is planar before supply of inflation gas and is inflated by supply of the inflation gas; an inflation portion (46; 125) provided in the airbag; a partition member (50; 50F; 50H; 50L; 50R; 50U) that partitions the inflation portion (46; 125) into at least an upstream inflation portion (47; 88; 91; 93; 97; 99; 101; 106; 108; 111; 112; 127) and a downstream inflation portion (48; 89; 92; 94; 98; 100; 102; 107; 113; 129); and a pressure regulating valve that is closed at an early stage of a supply period of the inflation gas to the inflation portion (46; 125), thereby restricting flow of the inflation gas from the upstream inflation portion (47; 88; 91; 93; 97; 99; 101; 106; 108; 111; 112; 127) to the downstream inflation portion (48; 89; 92; 94; 98; 100; 102; 107; 113; 129), and is opened from the middle of the supply period to cancel the restriction, wherein the partition member (50; 50F; 50H; 50L; 50R; 50U) has a length in a longitudinal direction that is longer than a length in a transverse direction perpendicular to the longitudinal direction when being tensioned to be planar upon inflation of the inflation portion (46; 125), and the pressure regulating valve includes: a slit-like inner opening that is provided on the partition member (50; 50F; 50H; 50L; 50R; 50U) and extends in the transverse direction of the partition member (50; 50F; 50H; 50L; 50R; 50U); and a pair of valve body portions (73, 74; 78, 79) that are provided around the inner opening and are selectively close to and separated from each other. a slit-like inner opening that is provided on the partition member (50; 50F; 50H; 50L; 50R; 50U) and extends in the transverse direction of the partition member (50; 50F; 50H; 50L; 50R; 50U); and a pair of valve body portions (73, 74; 78, 79) that are provided around the inner opening and are selectively close to and separated from each other.