The present invention provides a safe hyaluronic acid base material that is suitable for use in practical hyaluronic acid pharmaceutical preparations capable of distribution at room temperature and having such a low viscosity that injection is easy. The hyaluronic acid pharmaceutical preparations can reside in a joint cavity for a prolonged period of time while exhibiting analgesic effects. More specfically, there is provide a hyaluronic acid modification product in which hyaluronic acid and/or a pharmaceutically acceptable salt thereof is bounded to a block polymer selected from PEO-PPO-PEO, PPO-PEO-PPO, PEO-PLGA-PEO, PLGA-PEO-PLGA, PEO-PLA-PEO and PLA-PEO-PLA. The hyaluronic acid modification product, despite capability of distribution at room temperature and ease in handling because of the low viscosity, can have its viscoelasticity rapidly increased after injection into a living body, so that it is highly useful in treatment of joint diseases, aid in surgical operation, repair of tissue, etc. as a main ingredient of novel and practical hyaluronic acid pharmaceutical preparations.