Casiotron 46CS-27

Hey guys.

My grandfather recently gave me one of his old watches, a Casiotron 46CS-27 watch.

http://timerecs.com/imgs/a/b/u/z/y/rare_vintage_casio_casiotron_46cs___27_alarm_chrono_watch_xlnt_condition_1977_5_lgw.jpg

I really like the look and feel of it but when I opened it, it had no battery or label inside indicating what battery it uses. Does anyone know what type of battery it uses.

TY.