An apparatus and a method for determining a drowsy state include an information acquirer acquiring physiological information of a user, a mode determiner setting an operation mode among modes categorized according to variable applied to determine a drowsy state of the user. A threshold determiner sets a threshold for an application variable of the set mode based on data in a reference table stored in database (DB). A physiological index calculator calculates a measurement value for each of the application variables based on state information of the user acquired in real time from the information acquirer. A state determiner determines the drowsy state of the user by comparing a user table generated from the measurement value calculated for each of the application variables with the reference table.
