To provide a cellulose derivative excellent in thermoplasticity, water resistance and strength (elastic modulus, impact strength), a cellulose derivative, which is obtained by introducing a short-chain organic group (acetyl group), a medium-chain organic group having 3 to 5 carbon atoms and a long-chain organic group having 6 to 30 carbon atoms at the following substitution degrees DSs into a cellulose, is used: Short-chain organic group: 0.7≤DSSH≤1.5; Medium-chain organic group: 0.5≤DSME≤2.0; Long-chain organic group: 0.1≤DSLO