We have studied the self-propelled motion of a camphor disk on a 2,2'-bis(2-chlorophenyl)-4,4',5,5'-tetraphenyl-1,2'-biimidazole (o-Cl-HABI) molecular layer, which was developed on water, as a photomechanical sensing system. The o-CI-HABI dimer changed to its monomeric form upon UV light irradiation, and as a result, the surface pressure of the o-Cl-HABI molecular layer decreased. The reciprocating motion of a camphor disk in the absence of UV light irradiation was observed at A = 45 angstrom(2) molecule(-1), of which the surface pressure was, similar to 10 mN Random motion was observed under UV light irradiation at A = 45 angstrom(2) molecule(-1), of which the surface pressure was similar to 5 mN m(-1). Therefore, the nature of motion of a camphor disk changes depending on the photosensitivity of the o-CI-HABI molecular layer. We have discussed the mechanism of the change in the motion of the camphor disk in relation to the photoreaction of the o-Cl-HABI molecular layer with the surface pressure acting as the driving force.