To provide an opening implement for envelopes, allowing for a blade member to be easily exchanged.

The opening implement for envelopes is equipped with a case body 4, a blade member 6 which is movably stored inside the case body 4 and an operative member 8 which is movably supported inside the case body 4. The blade member 6 is structured so as to freely move in the oblique direction toward one side part 14 of the case body 4, between a storage position at least for its major part to be stored inside the case body 4 and a protruding position for its leading part to be protruded from the one side part 14 of the case body 4. An attachment and detachment mechanism provided relative to the blade member 6 generates a detachment-inhibiting state which inhibits the blade member 6 from detaching from the case body 4 and a detachment-cancellation state which cancels an inhibition of detachment of the blade member 6 from the case body 4, allowing for the blade member 6 to be easily exchanged.