Numerical control with tool life control function in consideration of changes of the processing conditions

A numerical controller determines a usage factor which represents the degree to which a tool of a machine tool in dependence upon machining conditions consumed lifetime, and calculates a tool life (accumulated consumption duration) that is consumed prior to each block based on the determined usage factor and a tool use for each Block. Is a command of a machining program read out from the block, a tool change command, a tool is selected having a tool life that does not end before the next tool change command on the basis of the accumulated consumption of time.