The liquid droplet forming device includes a liquid droplet discharger that discharges, as a liquid droplet, a particle suspension liquid in which on or more fluorescent particles are suspended; a light source for irradiating light onto the liquid droplet discharged from the liquid droplet discharger; a photodetector that receives fluorescence that is emitted by the one or more fluorescent particles included in the liquid droplet in response to the light as excitation light; and a particle number detector that detects a number of the one or more fluorescent particles included in the liquid droplet based on information from the photodetector.