The invention provides a medium additive, medium composition and a culture method and the like, capable of efficiently culturing cells or tissues in a well dispersed state, and further, permitting cell image analysis of the cells or tissues. The medium additive or medium composition contains agar, which preferably is a low molecular weight agar having a weight average molecular weight of 10,000-60,000. Using same, cells or tissues can be cultured in a well-dispersed state in a medium, and a proliferation promoting effect for the cells or tissues can also be obtained. In addition, the cells can be cultured in any of a floating state and a precipitated state by adjusting the concentration of the aforementioned agar.