1. An action estimating apparatus comprising: an image acquisition unit configured to repeatedly obtain a photographic image of an occupant of a movable object; a feature point detection unit configured to obtain feature point data indicating a position of a feature point of the occupant in the photographic image in response to obtaining of the photographic image by the image acquisition unit, the feature point being at least one predetermined point of a person's body; a model storage unit configured to store action estimation models each defining a position or a locus of the feature point in each specific action, which an occupant may perform; an action estimating unit configured to compare the feature point data obtained by the feature point detection unit with each of the action estimation models stored in the model storage unit to detect an estimated action, which is the specific action associated with the action estimation model, which is highest in a coincident degree; a definite action detecting unit configured to detect that a definite action is being performed by the occupant using a method different from the comparison between the feature point data and the action estimation model, the definite action being at least one specific action specified beforehand; a definite action model generating unit configured to generate a new definite action estimation model by modifying a position or a locus of the feature point, which constitutes a definite action estimation model, according to an in-action feature point, which is the feature point data obtained by the feature point detection unit when the definite action detecting unit detects the definite action performed by the occupant, the definite action estimation model being the action estimation model corresponding to the definite action; a non-definite action model generating unit configured to generate a new non-definite action estimation model using an in-action feature point according to a predetermined correspondence between a position or a locus of the feature point when the occupant performs the definite action and a position or a locus of the feature point, which constitutes the non-definite action estimation model, which is the action estimation model corresponding to a non-definite action, the non-definite action being a specific action other than the definite action; and a model update unit configured to perform model update to: update the definite action estimation model stored in the model storage unit to the new definite action estimation model generated by the definite action model generating unit, and update the non-definite action estimation model stored in the model storage unit to the new non-definite action estimation model generated by the non-definite action model generating unit.