A conventional stepper includes two pedals that can be pivoted up and down about horizontal axles.  The movement of the pedals is limited to vertical directions.  Therefore, a user can only exercise his or her legs. 
Another conventional stepper includes two pedals that can be pivoted about two inclined axles extended from a post.  The inclined axles and the post form a Y-shaped structure.  One of the pedals will be lifted and moved towards the post if the other pedal is trodden and moved away from the post, i.e., outwards.  Therefore, a user is forced to twist his or her waist while exercising his or her legs by treading the pedals.  The user uses a little energy to twist his or her waist because he or she easily keeps balance while moving the pedal downwards and outwards. 
The present invention is therefore intended to obviate or at least alleviate the problems encountered in prior art.