1. A program method of a semiconductor memory device, comprising: performing a program loop using a blind program operation so that threshold voltages of selected memory cells reach a first verification level; if at least one cell having the threshold voltage reached the first verification level is detected during the program loop, verifying whether there is a cell having the threshold voltage reached a second verification level higher than the first verification level; if, as a result of the verification, there is a cell having the threshold voltage reached the second verification level, continuously performing a program loop on memory cells having the first verification level as a target level and a program loop on memory cells having the second verification level as a target level; and if, as a result of the verification, there is no cell having the threshold voltage reached the second verification level, performing a program loop on memory cells having a target level higher than the first verification level, after programming the memory cells having the first verification level as the target level.