The present invention provides a method for producing a co-crystal including thiophanate-methyl and a triazole-based compound such as tebuconazole and a method for producing an agricultural and horticultural chemical preparation using the co-crystal as a raw material. As one example, the agricultural and horticultural chemical preparation can be obtained by a method including the steps of obtaining a sol including a co-crystal by stirring a suspension including thiophanate-methyl, a triazole compound such as a tebuconazole, a surfactant, a defoaming agent and water at a mixing degree of 0.1-1.0 while holding the temperature of the suspension at 60° C.; and adding auxiliary components such as silica, calcium carbonate and the like to the sol.