The speech synthesizing device acquires numerical data at regular time intervals, each piece of the numerical data representing a value having a plurality of digits, detects a change between two values represented by the numerical data that is acquired at two consecutive times, determines which digit of the value represented by the numerical data is used to generate speech data depending on the detected change, generates numerical information that indicates the determined digit of the value represented by the numerical data, and generates speech data from the digit indicated by the numerical information.