1. A navigation apparatus including a voice receiver to receive an instruction by voice input, and a voice recognizer to carry out voice recognition of the instruction received by the voice receiver, the navigation apparatus comprising: a recognition vocabulary comprehension level decider to decide a user comprehension level of a recognition vocabulary for instructions recognizable by the voice recognizer, from at least one of correction operation frequency and time-out frequency in an operation of recognizing the instruction which is carried out during the voice recognition by the voice recognizer and corresponds to the instruction; an operational transition determiner to determine an operational transition from a plurality of potential operational transitions in accordance with a decision result of the recognition vocabulary comprehension level decider, each of the potential operational transitions including a different number of input steps by which the instruction is voice-recognized, such that different input steps correspond to different subsets of the recognition vocabulary for each potential operation transition that includes multiple input steps; and an operational transition provider to provide the operational transition determined by the operational transition determiner, wherein the operational transition determiner, when determining the operational transition in accordance with the decision result output from the recognition vocabulary comprehension level decider, switches the operational transition thereby limiting an input content per step by increasing the number of input steps in a specific operational transition, or reducing the number of input steps by increasing an amount of information capable of being input per step in the specific operational transition.