A cell culture apparatus comprising: an observing unit configured to observe cells in a culture vessel; a measurement-position designating unit configured to designate a measurement position in the culture vessel observed by the observing unit; an image measuring unit configured to capture and record observation images at positions designated by said measurement-position designating unit; an image processing unit configured to calculate a confluent rate as a proportion of an area occupied by cells in an observation image, said area defined by a plurality of positions designated by the measurement-position designating unit; a time-based recording unit configured to record discrete variations with hour in the confluent rate of the cell calculated by the image processing unit; and a subculture-timing determining unit configured to determine a subculture timing of the culture vessel, based on the confluent rate calculated by the image processing unit and for a predetermined period of time, wherein the subculture-timing determining unit determines the subculture timing, based on a first threshold value (Ap) with respect to an average value of entire measurement areas on the observation image, and a second threshold value (Ap') with respect to at least one measurement area on the observation image, wherein the second threshold value (Ap') is larger than the first threshold value (Ap), wherein the measuring position in the culture vessel is the center of the culture vessel and is designated measurement area P0, and measurement areas P1 to P8 are set at equal intervals along a side of a square at positions separated from the measurement area P0 by at least a predetermined distance, the measurement-position designating unit is arranged to set at least one measurement area out of a straight line passing through the measurement area P0 and any one of the measurement areas P1 to P8. an observing unit configured to observe cells in a culture vessel; a measurement-position designating unit configured to designate a measurement position in the culture vessel observed by the observing unit; an image measuring unit configured to capture and record observation images at positions designated by said measurement-position designating unit; an image processing unit configured to calculate a confluent rate as a proportion of an area occupied by cells in an observation image, said area defined by a plurality of positions designated by the measurement-position designating unit; a time-based recording unit configured to record discrete variations with hour in the confluent rate of the cell calculated by the image processing unit; and a subculture-timing determining unit configured to determine a subculture timing of the culture vessel, based on the confluent rate calculated by the image processing unit and for a predetermined period of time, wherein the subculture-timing determining unit determines the subculture timing, based on a first threshold value (Ap) with respect to an average value of entire measurement areas on the observation image, and a second threshold value (Ap') with respect to at least one measurement area on the observation image, wherein the second threshold value (Ap') is larger than the first threshold value (Ap), wherein the measuring position in the culture vessel is the center of the culture vessel and is designated measurement area P0, and measurement areas P1 to P8 are set at equal intervals along a side of a square at positions separated from the measurement area P0 by at least a predetermined distance, the measurement-position designating unit is arranged to set at least one measurement area out of a straight line passing through the measurement area P0 and any one of the measurement areas P1 to P8.