An impact detector includes a circular weight, a case containing the weight, in which a transition path for the weight is formed, a planar elastic member extending in the case with a first end oriented in a direction in which the weight move upon an impact in a first direction, a fixing member fixing the planar member to the case. The planar elastic member includes a folded portion disposed on a second end of the planar elastic member, and a first arm and a second arm both extending from the folded portion. An end portion of the first arm holds the weight at an initial position, and the first arm deforms to release the weight upon the impact in the first direction. The fixing member includes a sandwiching portion sandwiching the folded portion and a contact portion in contact with both the first arm and the second arm.