近年投資信託のリスクが把握しにくくなっている．なぜならば，投資信託の組み入れ対象となる金融商品の仕組みが複雑になるとともにリスクの大きさが把握しにくくなってきたからである．そこで，投資信託の取引において，投資家が想定外のリスクを負うことを未然に防ぐため，分散投資規制という規制が設けられた．分散投資規制は，1つの資産への投資が過度に集中しないよう，1つの資産への投資額を投資信託純資産の一定割合以下にするものである．これまでに，人工市場を用いて，分散投資規制がファンダメンタル価格急落時に市場に与える影響について分析されている．しかし，金融市場が活況なとき，すなわち，ファンダメンタル価格が急騰するような状況において分散投資規制が市場に与える影響については議論されていない．そこで，本研究では人工市場を用いて分散投資規制がファンダメンタル価格急騰時の価格形成に与える影響について調査した．その結果ファンダメンタル価格急騰時には，ファンダメンタル価格が急騰した資産の取引価格が上昇することが分かった．そして，規制を受けるエージェントの割合が増加するにつれ，その傾向が段階的に弱まり，最終的にはファンダメンタル価格に収束しないことが分かった．また，ファンダメンタル価格急騰時と急落時に関して比較し調査した．その結果，ファンダメンタル価格急落時は急騰時に比べより影響を与えることを確認した．

In recent years the risk of mutual funds has become difficult to assess. This is because mutual funds have come to choose various assets, some of which may have high risk, and there may be some funds whose performances depend on dangerously much those of a part of the assets that the funds hold, and so on. Companies managing mutual funds are expected to perform risk control to prevent investors from taking unforeseen risk of funds. A related revision to the investment fund legal system in Japan led to establishing what is known as “the rule for investment diversification.” Some papers discussed that the rule for investment diversification affected price formation at the time of a market crash; however, we could not find that it affects price formation at the time of a soaring market. In this paper, we investigated that the rule for investment diversification affected price formation in financial markets where two types of investors who followed the rule and did not follow it participated at the time of a soaring (crashing) market that was caused by the bulge (collapse) of an asset fundamental price using agent-based simulations and discussed the difference between these effects. As results, we found that, in two-asset markets where two types of investors who followed the rule and did not follow it participated, when one asset fundamental price soared, the rule for investment diversification prevented its price rising, and when one asset fundamental price collapsed, the rule promoted the other's price to decline.