Needle-like microcrystals growing after photoisomerization from an open-ring (1o) to a closed-ring (1c) isomer in a diarylethene 1 film were studied with an optical microscope. The photoisomerization and the crystal growth in the diarylethene 1 film were not measured in situ by scanning electron microscopy but measured with an optical microscope. The film set on the stage of an upright/inverted microscope was exposed to UV and visible (yellow) light using a Xe lamp in situ to induce the reversible photochromic reactions; optical and spectroscopic images of the diarylethene 1 film were also observed in situ using a halogen lamp. At 1 h after the photochromic isomerization reaction to 1c under the UV light, needle-shaped microcrystals were observed in the inplane radial direction from the outline of island-shaped film, and they gradually grew. After incubation for 41 h in the dark at room temperature, the film was exposed to visible light for the reverse isomerization to 1o. It was confirmed from absorption spectra, and the crystals simultaneously disappeared. The optical and spectroscopic imaging with a microscope was found to be a powerful tool for in situ observation of the photochromic reaction and crystallization processes.