1. A work recognition device, comprising: a whole-state observation time determination unit configured to determine, as a whole-state observation time, from among observation times of a plurality of first observation values generated by a speed sensor to represent respective movements of a hand of a user in chronological order, an observation time of observing a first observation value representing a predetermined movement of the hand, based on magnitudes of the plurality of first observation values and an angular difference between the plurality of first observation values; and an observation value selection unit configured to select a second observation value corresponding to the whole-state observation time determined by the whole-state observation time determination unit, from among a plurality of second observation values generated by an imaging unit to represent respective states in work of the user in chronological order, wherein the whole-state observation time determination unit includes: a magnitude estimation unit configured to estimate a magnitude of an acceleration that is obtained by subtracting a gravity acceleration from each of the accelerations generated by the speed sensor; an angular difference calculation unit configured to calculate, for each of the accelerations generated by the speed sensor, an angle between (a) one acceleration at a corresponding observation time and (b) another acceleration at an observation time prior to the corresponding observation time; and a time determination unit configured to determine, as the whole-state observation time, the corresponding observation time, when (a) a magnitude of the one acceleration which is estimated by the magnitude estimation unit and (b) the angle calculated by the angular difference calculation unit show a predetermined pattern.