Officials in prison fear that the 74-year-old may try to kill himself.
His cellmate is under orders to report odd behaviour, reports The Sun.
Last week, Fritzl was convicted over his daughter's 24-year rape ordeal.
His jail door sign in St Poelten, Austria, reads 'Date of Release: Death.'
Fritzl was jailed for murder, rape, enslavement, coercion and incest after locking Elisabeth in his cellar for 24 years and fathering seven children by her, six of whom survived.