An impact detector (100; 600; 900; 1100) comprising: a weight having a circular periphery; a case (400,200; 200A; 200B; 1200) in which the weight is contained, an interior of which defines a transition path through which the weight moves when the case (400,200; 200A, 200B, 1200) receives an impact of a predetermined magnitude in a first direction; an angled U-shaped planar elastic member (310; 320; 620; 1410; 1420) disposed on a first side of the case (400,200; 200A, 200B, 1200), extending in the case (400,200; 200A, 200B, 1200) with a first end portion oriented in a direction in which the weight moves when the case (400,200; 200A, 200B, 1200) receives the impact in the first direction, the angled U-shaped planar elastic member (310; 320; 620; 1410; 1420) including, a linear folded portion (313; 323; 623; 1413; 1423) disposed on a second end portion of the planar elastic member (310; 320; 1410; 1420) opposite to the first end portion of the planar elastic member (310; 320; 620; 1410; 1420), a first arm (311; 321; 621; 1411; 1421) bent and extending from a first end portion of the linear folded portion (313; 323; 623; 1413; 1423), including an end portion holding the weight at an initial position in the transition path, the first arm (311; 321; 621; 1411; 1421) deforming to release the weight when the case (400,200; 200A, 200B, 1200) receives the impact of the predetermined magnitude in the first direction, and a second arm (312; 322; 622; 1412; 1422) extending from a second end portion of the linear folded portion (313; 323; 623; 1413; 1423) opposite the first end portion from which the first arm (311; 321; 621; 1411; 1421) extend; and a fixing member (241,267,461; 242,268,462) effective to fix the planar elastic member (310; 320; 620; 1410; 1420) to the case (400,200; 200A, 200B, 1200), the fixing member (241,267,461; 242,268,462) including, a sandwiching portion (241,461; 242,462) effective to sandwich a center portion of the linear folded portion (313; 323; 623; 1413; 1423) of the planar elastic member (310; 320; 620; 1410; 1420) from both sides, and a contact portion (241b,241c; 242b,242c) effective to prevent the planar elastic member (310; 320; 1410; 1420) from moving in the direction in which the weight moves by contacting the first arm (311; 321; 621; 1411; 1421) and the second arm (312; 322; 622; 1412; 1422) of the angled U-shaped planar elastic member (310; 320; 620; 1410; 1420). a weight having a circular periphery; a case (400,200; 200A; 200B; 1200) in which the weight is contained, an interior of which defines a transition path through which the weight moves when the case (400,200; 200A, 200B, 1200) receives an impact of a predetermined magnitude in a first direction; an angled U-shaped planar elastic member (310; 320; 620; 1410; 1420) disposed on a first side of the case (400,200; 200A, 200B, 1200), extending in the case (400,200; 200A, 200B, 1200) with a first end portion oriented in a direction in which the weight moves when the case (400,200; 200A, 200B, 1200) receives the impact in the first direction, the angled U-shaped planar elastic member (310; 320; 620; 1410; 1420) including, a linear folded portion (313; 323; 623; 1413; 1423) disposed on a second end portion of the planar elastic member (310; 320; 1410; 1420) opposite to the first end portion of the planar elastic member (310; 320; 620; 1410; 1420), a first arm (311; 321; 621; 1411; 1421) bent and extending from a first end portion of the linear folded portion (313; 323; 623; 1413; 1423), including an end portion holding the weight at an initial position in the transition path, the first arm (311; 321; 621; 1411; 1421) deforming to release the weight when the case (400,200; 200A, 200B, 1200) receives the impact of the predetermined magnitude in the first direction, and a second arm (312; 322; 622; 1412; 1422) extending from a second end portion of the linear folded portion (313; 323; 623; 1413; 1423) opposite the first end portion from which the first arm (311; 321; 621; 1411; 1421) extend; and a fixing member (241,267,461; 242,268,462) effective to fix the planar elastic member (310; 320; 620; 1410; 1420) to the case (400,200; 200A, 200B, 1200), the fixing member (241,267,461; 242,268,462) including, a sandwiching portion (241,461; 242,462) effective to sandwich a center portion of the linear folded portion (313; 323; 623; 1413; 1423) of the planar elastic member (310; 320; 620; 1410; 1420) from both sides, and a contact portion (241b,241c; 242b,242c) effective to prevent the planar elastic member (310; 320; 1410; 1420) from moving in the direction in which the weight moves by contacting the first arm (311; 321; 621; 1411; 1421) and the second arm (312; 322; 622; 1412; 1422) of the angled U-shaped planar elastic member (310; 320; 620; 1410; 1420).