The photochemical reaction pathway of silacyclopropenylidene (c-C2H2Si) has been investigated using the eight electrons in eight orbitals complete active space with the 6-311++G(3df,3pd) basis sets. The mechanism of drastic structural change in the reaction of c-C2H2Si and the difference from the reaction of cyclyopropenylidene (c-C3H2) are elucidated. The photochemically active relaxation path of c-C2H2Si on the S-1 excited-state potential surface leads to an S-1/S-0 conical intersection where the photoexcited system decays nonradiatively to So. The relaxation of c-C2H2Si on the S-1 surface causes the cleavage of the Si-C single bond, while that of c-C3H2 causes the cleavage of the C-C double bond. The difference in photochemical cleavage sites is well explained by the difference in the electronic nature of the S-1 excited state. In the dark reaction following the relaxation on the S-1 surface, hydrogen migration from carbon to silicon occurs together with ring opening at the Si-C bond.