1. A song search system that searches for desired song data from among a plurality of song data stored in a song database, the song search system comprising: a song-data-input means of inputting said song data; a characteristic-data-extraction means of extracting physical characteristic data by performing a Fast Fourier Transform on a set frame length and by calculating the power spectrum from song data input by said song-data-input means; an impression-data-conversion means of converting the physical characteristic data extracted by said characteristic-data-extraction means into impression data determined by human emotion; a memory-control means of storing impression data converted by said impression-data-conversion means in a song database together with song data input by said song-data-input means; a keyword set means of setting song data to correspond to one or more keywords; a song-mapping-display means of displaying a song map comprised of neurons each representing song data; a keyword-display means of displaying the one or more keywords corresponding to particular song data when the neuron representing the particular song data displayed on said song-mapping-display means is clicked on; an impression-data-input means of inputting impression data as search conditions; a song search means of searching said song database based on impression data input from said impression-data-input means and the one or more keywords; and a song-data-output means of outputting song data found by said song search means, wherein said impression-data-conversion means uses a pre-learned hierarchical-type neural network to convert characteristic data extracted by said characteristic-data-extraction means to impression data determined according to human emotion.