This invention relates to a so-called "keyless" type button operated combination lock which can be unlocked by a button operation and locked by a handle operation. A number of buttons exposed on the front side of a case can be set in a combination and a key plate that engages with a bolt lock and keeps it in the locked state is unlocked by setting the buttons, and actuator keys to which they are attached, to a combination but prevents unlocking if the unlocking operation is erroneously carried out. These keys are made in the same shape and size, and the set positions for the memory state and the non-memory state can be interchanged by reversing key positions. These keys are loosely inserted into corresponding slits in the key plate and permit the selection of various operating conditions in conjunction with the key plate slits. When the keys are brought into the "ON" state, they are detachably hooked to the lock plate. The handle, which is operative in the interlocking arrangement with the bolt lock, permits the key plate and the lock plate to simultaneously operate under engagement with each other via a cam pin. Another cam pin engages only with the above-mentioned cam pin and with the lock plate. These cam pins are interchangeable with each other so that the lock of the invention can be fitted to the door as a right-handed type or a left-handed type.