The present paper reviews results of series of the studies pertaining to ecology and control of bacterial canker of tomato caused by Clavibacter michiganensis subsp. michiganensis. The infection of this disease took place when a population density of the causal bacteria was more than 10⁶ cfu/g of tomato leaves, while there was no disease under the density of less than 10³-10⁵ cfu/g. Occurrence of bacterial canker in tomato was severer in an open field than in a protected field from rainwater. It may be concluded that multiplication and dissemination of causal bacteria are augmented under a rainfall condition in an open field. This result suggests that the cultivation of tomato plants under plastic films to avoid rainfalls be one of the practical methods for controlling bacterial canker. The selective medium, SMCMM which was developed for isolating causal bacteria from plant materials or from soils, was useful for an ecological study of this disease, especially for checking a population density of causal bacteria. It was recognized that the ELISA method could be used for detection of C. m. ssp. michiganensis and for the rapid diagnosis of the disease.