1. A fluorescence detecting device using an intensity-modulated laser beam, which irradiates an object to be measured with a laser beam to receive fluorescence generated by the object to be measured, and carries out signal processing of a fluorescence signal obtained when receiving the fluorescence, comprising: a laser light source section that outputs the laser beam with which the object to be measured is irradiated; a light receiving section that outputs the fluorescence signal of the fluorescence generated by the object to be measured which is irradiated with the laser beam; a light source control section that generates a modulation signal having a given frequency in order to time-modulate an intensity of the laser beam that is output from the laser light source section; and a processing section that calculates, by using the modulation signal, a fluorescence relaxation time of the fluorescence of the object to be measured based on the fluorescence signal that is output from the light receiving section by irradiating the object to be measured with the time-modulated laser beam, wherein the processing section obtains a phase delay with respect to the modulation signal of the fluorescence signal to calculate the fluorescence relaxation time, wherein the light source control section uses as a pulse control signal a coding sequence signal that is selected from a plurality of coding sequence signals which have signal values of one bit coded with a given length and are orthogonal to each other, and sets and controls on/off of output of the laser beam so that an on-time of the output of the laser beam from the laser light source section is longer than one cycle time of the time modulation of the laser beam, and wherein the processing section calculates the fluorescence relaxation time and identifies the fluorescence from the object to be measured, by using the coding sequence signal based on a light receiving signal that is output from the light receiving section.